A computer virus is a malicious program that is loaded onto your computer, often without your consent, and sometimes without your knowledge. Computer viruses are small programs that are designed to interfere with normal computer operation. Viruses can cause a range of damage to your computer, from mild annoyance (popups), to significant loss of data and system corruption.
If your computer is performing slower than normal or acting erratically and doing unusual things, you may have a virus!
The best thing you can do for your computer is make an effort to prevent viruses from attacking it in the first place. Always be sure to have an up-to-date anti-virus program running on your computer. There are many free solutions on the Internet.