It sounds like you have a FAT virus , once a virus gets into the FAT {file allocation table}
it replicates and stays there most virus programs don't scan the FAT as it isn't the files on your computer but the way the files are seen and used, one byte of information corrupted and the computer can't recognize the file, the file is clean, but the way its read and processed isn't, I personally don't use any of the virus programs offered but use one from my internet provider that scans data before it even is seen by my computer.
Bigfish it is freeware has a warning on there site that they are not responsible because anyone can download and upload games to it

Back up your files, run the best virus scan and prepare to reload windows, you will have to scrub it completely clean...run the fdisk program in windows from bootup, and delete the FAT completely then build another and format the HD ... then reload windows...restore will not set the FAT back only the files.