There's not exactly compatibility between versions of Windows, either. Want your XP app to run on Win7? I hope you weren't planning to write to your install directory, since that will require a permissions change on Win7. That XP app is probably 32-bit, too, and if you are on 64-bit Win7 I hope the app isn't looking for "C:\Program Files" to find itself, since it's now in "C:\Program Files (x86)". Why is it still not working? Maybe one of the API calls works differently, better try running in XP compatibility mode.<p>Simple stuff works great in new versions of Windows, but my experience is that if you expect it to work, you'd better test it on every version of Windows, because there will be version specific bug on one of them.