Computer viruses are insidious, sneaky and--like their biological counterparts--constantly mutating. If you think your computer has been infected, the only safe course of action is to use a good anti-virus program.
Step1
As soon as you suspect that your computer has a virus, remove your computer from any networks it might be on, as well as from the Internet, so that you don't inadvertently spread the bug to others. Unplug your network cable if you have to.
Step2
If you have virus-scanning (anti-virus) software installed, run it.
Step3
If you don't have anti-virus software, you'll need to obtain some. If you can't get it from a network administrator or download it from an uninfected computer, you can mail-order it from a retailer.
Step4
Start your computer (still not connected to a network) and follow the instructions that came with the anti-virus software.
Step5
Keep running the virus-scanning software until your computer comes up clean.
Step6
Reconnect your computer to the Internet and check with the anti-virus software's publisher to make sure you have the latest updates. If not, download them now.
Step7
After updating the anti-virus software, run it again until your computer comes up clean.

Warning:
There's no substitute for prevention. Good anti-virus software more than pays for itself as long as you keep it up-to-date.
Common sense also goes a long way toward keeping your computer clean. Never open an attachment from someone you don't know, and be suspicious of odd attachments from people you do know (a virus may have mailed itself to you from their computer)