Technical issues with Heroes III.

Hello! I have a problem: completed campaigns are not being saved. That is, autosaves exist, regular saves also exist, and the result is recorded in the table – but the campaign either stops being marked as completed when the game is launched next time, or it simply disappears if you navigate through the game menu. Consequently, new campaigns cannot be unlocked, which is disappointing. I have tried different discs and operating systems (Windows XP and Windows 7). I can't figure out what the problem might be.