Go to Control Panel and delete any anti-virus program that you have on your computer. Then, go to the first web site below (scroll down a little until you see the downolad button under the Free column) and download the AVG Anti-Virus Free Edition and install it. The setup is quick and painless. Be sure to download all the updates to the AVG program after installation. This is done within the program. Then, do a complete system scan. I have been using AVG AV Free for the last five years and have never had a virus on my computer. It's a great AV program.
Get a couple of Spyware programs if you don't have them already and scan your system for spyware. No one program will get rid of all spyware, usually not.
How much RAM memory is in your system? If you're running Windows XP, you should have a minimum of 512 MB of RAM. Your system will run considerably faster if you increase your system RAM to 1 GB