Microsoft Security Essentials is my preferred free anti virus solution which I have written about before.
Microsoft Security Essentials
Now if you have a badly infected computer , what should you do?
If you are that badly infected and do not want to format the hard drive and reinstall the operating system ( which should be your first choice) then Microsoft have released the Microsoft Safety Scanner
If you can , go to this page and it will enable you to download and make a boot disc which will check your computer for virus infection.
The boot disc is a disc which you put into your computer then restart and it will check your computer for virus infections.
Remember you may have to make the discs from a good computer that is not infected.
Microsoft also have the Microsoft Standalone System Sweeper which will scan your system for rootkits which are not always visible to virus scanning.