Virus? Windows XP (prog. name) is not a valid Windows image.

[GameOver1]

Hey guys, well I'm pretty sure I've acquired a virus somehow. I started to notice the problem when every program I attempted to open came up with the error:

C:\Program Files\(program location)\(program name).exe is not a valid Windows image. Please check your installtion diskette.

I scheduled a boot scan with Avast and it found various infected files and deleted them, however the problem still persists in fragments. Only certain programs give this error now.

For example: when I try to download a program like Adaware, it says it is not a valid Windows image.

Any ideas on what to do? I figured the boot scan would find the problem and eliminate it but I learned otherwise. Virtually I'm stuck.