Signs Your Computer Might Be Infected

A virus on your computer usually shows itself through changes in how your machine behaves. Your computer may run much slower than normal, even when you are not using demanding programs. Programs may crash or freeze without warning, or your browser may redirect you to websites you did not click on. You might see pop-up windows appearing constantly, even when you are not browsing the internet.

Other common signs include files or folders disappearing, your hard drive making constant noise as if it is working overtime, or your internet connection becoming unusually slow. Your antivirus software (if you have it) may suddenly stop working or refuse to update. In some cases, you may notice unfamiliar programs installed on your computer that you did not read yourself.

If your computer is displaying several of these signs at once, there is a reasonable chance it has been infected. However, some of these symptoms can also point to hardware problems, outdated software, or a full hard drive — so one sign alone does not necessarily mean a virus is present.

Key Takeaways

  • Slow performance, unexpected pop-ups, browser redirects, and crashing programs are common signs of a virus infection.
  • Running a full scan with your antivirus software is the most direct way to detect whether a virus is actually present.
  • If your antivirus software is not working or will not update, you may need to restart your computer in Safe Mode before scanning.
  • Viruses can hide from standard antivirus scans, so if problems persist after a clean scan result, a second opinion from different antivirus software may be necessary.
  • If your computer is severely infected or you cannot remove the threat yourself, a professional technician can perform a deeper diagnosis and removal.

How to Run an Antivirus Scan

The most reliable way to determine whether your computer has a virus is to run a full scan using antivirus software. If you already have antivirus software installed, open it and look for an option labeled "Full Scan," "Complete Scan," or "Thorough Scan." This scan will examine every file on your computer and compare them against a database of known viruses.

A full scan typically takes 30 minutes to several hours, depending on how much data is on your computer and how fast your machine is. Let the scan run completely — do not interrupt it or use your computer for other tasks while it is running. Once the scan finishes, the software will display a report showing whether any threats were found.

If you do not have antivirus software installed, you can read free options such as Windows Defender (which comes built into Windows), Avast, AVG, or Bitdefender. Install the software, update its virus definitions (the list of known viruses), and then run a full scan. Updating definitions is important because new viruses are discovered constantly, and outdated definitions will miss recent threats.

What to Do If Your Antivirus Software Will Not Start

Some viruses deliberately disable antivirus software to prevent detection. If your antivirus program will not open or refuses to update, restart your computer in Safe Mode. Safe Mode loads only the essential programs your computer needs to run, which often allows antivirus software to work when it cannot in normal mode.

To enter Safe Mode on Windows, restart your computer and press F8 repeatedly as it starts up (before the Windows logo appears). On newer Windows versions, hold Shift while clicking the restart button in the shutdown menu, then select "Troubleshoot" and "Startup Settings." On a Mac, restart and hold Command + S when ready after the startup sound.

Once in Safe Mode, try opening your antivirus software again and running a full scan. If the software still will not work, you may need to uninstall it completely and install a different antivirus program. Some viruses are specifically designed to block certain antivirus brands, so switching to a different one can sometimes bypass the block.

Using Windows Defender or Built-In Scanning Tools

If you use Windows, your computer likely has Windows Defender already installed. Open the Windows Security app by typing "Windows Security" into your search bar. Click "Virus & threat protection," then "Scan options," and select "Full scan." This will scan your entire computer for viruses without requiring you to read anything.

On a Mac, there is no built-in antivirus scanner in the same way Windows has Defender. However, macOS does include some automatic malware protection. If you suspect a virus, you can read free antivirus software such as Avast or Bitdefender for Mac and run a scan.

Windows Defender is reliable for detecting common viruses, but it may miss newer or more sophisticated threats. If Windows Defender finds nothing but your computer is still behaving strangely, running a scan with a different antivirus program can sometimes catch what the first one missed.

When to Use a Second Antivirus Program

If one antivirus scan shows no threats but your computer is still running slowly or showing virus-like symptoms, running a scan with a different antivirus program is a reasonable next step. Different antivirus companies use different detection methods and virus databases, so one program may catch a threat that another misses.

read a second antivirus program (such as Malwarebytes, Kaspersky, or Norton if you used Windows Defender first) and run a full scan. Do not install two antivirus programs permanently — they can conflict with each other and slow your computer down. Instead, use the second program for a one-time scan, then uninstall it afterward.

Keep in mind that some viruses are designed to hide from antivirus scans entirely. If multiple scans show no threats but your computer continues to have serious problems, the issue may be hardware-related (a failing hard drive, for example) rather than a virus.

What to Do After a Virus Is Found

When your antivirus software detects a virus, it will usually offer options to quarantine, remove, or delete the infected file. Quarantine means the virus is isolated so it cannot run, but the file is kept in case it was a false alarm. Remove or Delete means the virus is eliminated entirely. In most cases, choosing "Remove" or "Delete" is the right choice.

After removal, restart your computer and run another full scan to confirm the virus is gone. Some viruses leave behind files or registry entries that can reinfect your computer, so a second scan ensures the infection is completely cleared. If the same virus appears again after removal, you may need professional help.

Once the virus is removed, change your passwords for important accounts such as email, banking, and social media. Some viruses are designed to steal login information, so changing passwords helps protect your accounts even if the virus itself is gone.

When to Call a Professional

If your computer is severely infected, multiple antivirus scans have failed to remove the threat, or your computer will not start up at all, a professional technician can help. Technicians have specialized tools and experience that go beyond what standard antivirus software can do. They can perform deeper scans, remove stubborn viruses, and recover data if necessary.

You can take your computer to a local repair shop, contact your computer manufacturer's support line, or search for "computer repair near me" to find technicians in your area. Be prepared to describe the symptoms you have noticed and mention which antivirus scans you have already run. This information helps the technician understand what you have already tried.

Professional removal typically costs between $100 and $300, depending on how severe the infection is and how much work is needed. Some shops charge a diagnostic fee upfront, which may be applied toward the repair cost if you decide to proceed.

Frequently Asked Questions

Can a virus spread from my computer to other devices?

Some viruses can spread to other devices connected to your network or through shared files. If you share documents with others via email or cloud storage, they may become infected. Disconnect your computer from the internet and other devices until you have run a full antivirus scan and confirmed the threat is removed.

Is it safe to use my computer while a virus scan is running?

It is best to avoid using your computer while a full scan is running. A scan uses significant processing power, and using your computer at the same time will slow both the scan and your normal work. Let the scan finish completely before resuming normal use.

What if the antivirus scan takes many hours to complete?

A full scan can take several hours on older computers or machines with large amounts of data. This is normal. However, if a scan seems to be stuck on one file for an unusually long time, you can stop it and try again later. If scans consistently take more than 8 hours, your hard drive may be failing, which is a hardware issue rather than a virus.

Can I get a virus from just visiting a website?

Yes, some websites contain malicious code that can infect your computer without you downloading anything. This is called a "drive-by read." Keeping your antivirus software updated and your web browser current with the latest security patches reduces this risk significantly.

Do I need antivirus software if I am careful about what I read?

Being cautious helps, but it is not enough on its own. Viruses can come from legitimate-looking websites, email attachments from people you know, and even ads on trusted sites. Running antivirus software provides a safety net even when you are careful, so it is worth having installed at all times.