Signs & Symptoms of a Virus Presence in a Computerby Aaron Parson
Even on a computer with anti-virus software, viruses and other malware may slip by and affect how your system operates. If your computer starts behaving oddly, slows down or pops up unfamiliar programs or messages, you should run a full virus scan, even if your anti-virus program has not alerted you to a problem.
All computers slow down over time as you install more programs, but a sudden, severe, long-lasting slowdown can indicate a virus. Whether the slowdown occurs while booting, while opening programs or while waiting for websites to load, it could come from a virus hijacking your machine's processing power or Internet bandwidth.
Frequent Crashes and Freezes
A program occasionally freezing or your computer crashing does not necessarily indicate a virus -- most of these errors occur due to buggy software. If your computer freezes or crashes multiple times a day, however, or the crashes start to occur in numerous different programs, a virus may be to blame. Viruses can cause crashes by altering or deleting necessary system files. If a virus scan turns up empty, your frequent crashes might indicate a failing piece of hardware instead.
Excessive Drive or Processor Use
It's normal for your hard drive to occasionally process data even when you aren't using your computer, but if your hard drive continues to churn away for hours at a time, a virus may be running in the background. Viruses that use up your CPU's processing power can cause your computer's fan to run loudly continuously. To check CPU use, right-click the taskbar and click "Task Manager." Press "More details" if you don't see any tabs in the Task Manager window, then check the "CPU" percentage. If this number remains elevated for several minutes -- over 20 percent without any programs running -- you might have a virus. Programs that fail to quit correctly can also cause high CPU use, which you can solve by rebooting or selecting them and pressing "End task."
Unfamiliar Programs and Pop-Ups
Many websites use ads legitimately, but if you suddenly see an increase in ads, or if ads appear while not you're browsing the Web, you might have a virus. Viruses can also manifest as unfamiliar programs appearing in your taskbar or running when you boot your computer. In most cases, these symptoms indicate adware -- unwanted, but non-malicious programs -- rather than a true virus, but some may also behave as viruses, affecting computer performance or hijacking your Web browser. If your anti-virus software doesn't detect these programs, install and run anti-malware software.
Information in this article applies to Windows 8. It may vary slightly or significantly with other versions.