Modified exe. file

I can install new software but when I run it for the first time I get an error message which briefly says "the applications executable file has been modified". Norton Internet Security crashed last week and I've had this problem since. C: drive has been checked for viruses and all seems ok.
Any ideas anyone or is it the dreaded Format C:!!

Jamon--If this is happening with all .exe files try ExeFix
http://home.earthlink.net/~rmbox/Reticulated/Toys.html
If only with one program, tell us which one, and from what source you downloaded/installed it. You could try uninstalling and reinstalling, especially if you have an alternative source.