My ancient (but works great!) Act! 6.03 program files somehow got corrupted yesterday. Tried to reinstall using my Act! 6 CD, but kept getting an error claiming that setup.exe (or autorun.exe) programs on the CD were incompatible and would not run on Win8.1 64-bit PC. I copied all of the files on the CD onto my HD and tried to run both of those programs using Windows Compatibility set to Windows XP (SP3). Still would not run.
Solved the problem by downloading the ACT!603.exe file from Sage website.