Oh Oh....what Is Happening Here?

[buddytod]

System= Athlon XP 2.4, 512mb mem, ABIT KD7 series mobo, Geforce 4 Ti4200, Sb Audigy, 80gb HDD, Win XP (problem occurred whilst using win ME)
This system has been working fine until 2 weeks ago (whilst still using ME).
Suddenly, several programs would not start anymore! Some just displayed the hour glass for a few seconds...then nothing, others gave a message saying that the file contents had altered. Examples are:- Smartripper, Clonecd, Musicmatch, Battlefield 1942, and this morning....Kazza!!!!
Having said that, there are a large number of other games/applications that are not affected. I formatted the drive and installed XP and it still continues. My son's computer, which is networked to mine, has now given a similar problem with Battlefield 1942. I have noticed that if I copy the good working .exe file for Battlefield, and store it in another folder, I can paste it into the battefield folder when the problem occurres and it then starts up ok, (for a while)! This seems to suggest that the contents of the .exe file are being altered. I suppose the obvious answer is some sort of virus.....but why only some applications and can it survive a format with fresh install. If so, what should I do? Any help greatly appreciated!

[balamm]

Duties are not performed for duty's sake, but because their neglect would make the man uncomfortable. A man performs but one duty - the duty of contenting his spirit, the duty of making himself agreeable to himself.
Mark Twain