Most of the time it happen with us any dangerous virus enters in our system. some time antivirus also found unable to remove that virus , in such case generally we format our system and lose our all data but it is not a best solution , if any virus not removed by antivirus you can still remove that virus with the help of system restore tool. System restore tool put your computer date in past date. you loose something but it is better than format your system.follow the following steps to use system restore tool.
1. Click start ==>Program==>Accessories==>system Tool==> Store
2. You will get window click on " Next"
3. Select the date on which you want to restore your computer then Click on Next.
4. After completion of restore process restart your computer now you have store your computer in back days. you will loose everything which you have installed between current and past date virus as well.