A computer virus is a computer program that can copy itself and infect a computer. The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware.

You find out if you have one by running antiviral scans.

You should have one and only one active protector like Avast.
You should have another two scans installed like Spybot Search and Destroy or A2 free. You should update these weekly and run them periodically.
Often these programs can remove them or identify them and let you get specific programs to remove them.

If you don't have a anti-software to 'alert' you and fix it, your computer would start giving out 'bad behavior'..Eating away important files until its 'dead'