AI in Might & Magic: Heroes VI

Where do you get these distributions, or is it because of Windows 8? My AI opponents are terrible.

It’s hard to say; perhaps you’re not manually setting up the AI opponents. Instead, you’re letting the computer do it for you.
You need to choose so that all the heroes are different, and then, one by one, set their AI to “difficult” by clicking on this section. Also, set the game difficulty to the highest level so that you start with only 2500 gold.
Personally, it’s not easy for me to complete the game without loading old saves. But that’s probably because I try not to study the map too thoroughly.