I have to delete the .exe, then run the installer every time I play Fallout 1 or 2. After I install the game, it will launch properly. If I exit within a few minutes, and run again, it's fine. However, after about 20-30 minutes after an install, if I try to launch the game I'll get an error about it not being a valid Win32 application.
I'm running Vista 32-bit.

try renaming the .exes?