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I had another thought, though I don't know if it will help. After I installed that Ultimate Windows version, my D drive disappeared. That is, in Disk Management, it was listed as "unallocated" or something like that. I had to save all the necessary files using R-studio, and they were duplicated or tripled during the saving process. At that time, I had Heroes installed on the D drive. Of course, I didn't save them because I can reinstall them. And after this problem (my Windows crashed, and I had to reinstall it, and after the installation, I found out that the D drive was gone), Heroes stopped launching. Maybe some files are corrupted somewhere, or something like that? What do you think, is that possible? Or is it not related?

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Maybe that's why it launches in XP Mode, because there's a virtual drive, and nothing interferes with it?

Added 2 minutes later
Here are some possible explanations: faulty hardware, a bad Windows installation (because Service Pack 1 can't be installed), a worn-out drive with the game, a conflict between the OS and the copy protection system (although it should be deactivated in version 1.6), user error, etc. This is what I was told on another forum. I'm writing this with my clumsy user hands