The issue was chiefly that XP didn't enforce good programming practices (storing user info in /Program Files/ was rife and one of many issues). Introducing the

The issue was chiefly that XP didn't enforce good programming practices (storing user info in /Program Files/ was rife and one of many issues). Introducing the UAC security structure meant that suddenly all of these applications tried to do things that triggered UAC elevation prompts. This was addressed twofold, as far as I understand: 1) Application developers were railroaded into best practices and forced not to do terrible design ideas, and 2) MS introduced a whole load of compatibility features such as virtualising folders transparently so when bad apps still tried to do bad things, it was securely captured and diverted around requiring a UAC elevation prompt. Regarding the stability issues, Vista's driver model was new when compared to XP. Naturally, the number of bugs in drivers increased initially but slowly settled down. Much of the '7 is more stable / faster than Vista' groupthink is the result of bad drivers getting better in the intervening years.