Strange one, I would try as mel says.
I believe you can do a drive scan - type CHKDSK into google there is a few different options I believe you can run you could try the /f one but could be worth waiting for Entwi.
Also I believe you can only Run one Anti Virus at a time.
Here is a good list / Link of FREE Programs for various Anti-Vir,Spy,Mal Ect
http://www.webuser.co.uk/forums/show.../page/0#219333
Good luck I hate PC's Sometimes when they start plating up. But I suppose that half the fun.