Heroes 3 issues under XP or Vista.

I haven't noticed any problems with other programs or games. Everything seems fine with the hardware; the computer doesn't overheat, there's plenty of memory, and there's ample disk space. DirectX is updated. The only thing I suspect, although it might not be the cause, is the video driver. I'll need to check that somehow.

The problem is resolved; it was all due to the video driver. For some reason, the version of the driver I had (which was quite old) didn't work well with the game.