XP was the worst when it came out. It had the most flaws in any release version of a Windows based OS to date. Even before they could push SP1 out there was well over 1,000 flaws found in their code.
I didnt adopt XP till well after SP2 was released due to the fact that there was more flaws than patches being released for the OS.
Vista was a bit of a resource hog, but at least it had only a handful of the flaws XP did.
The reason why people like Win7, it has the speed of XP and the security of Vista.