Viruses are going around allover
There is one right now I seen several times it installs something that looks like Antivirus software that does nothing but break your computer. It just turns off services you need to use your computer. Kills mail the browsers network services on various ports etc. The most recent is called AV Protection 2011 Do not buy anything from these clowns the software you get only breaks the computer more than it is already and costs 29 dollars in the process. It turns into a Zombie computer.
Avast Antivirus is fine Use the free one
You should know about when it happened. Do a System Restore to a point in time prior to that event.
If its a bad one you may need to start the computer in safe mode.
Hold Down the F8 button whilst turning on the computer.
After it boots into SAFE Mode
Choose Start > All Programs > Accessories > System Tools > System Restore
You get something that looks like a calendar. Select restore point prior to that event. The computer will reboot when it's complete