XP is actually built on the NT5 kernel, which is what Windows 2000 was based on. Out of the NT kernels, 5 has been the most stable by far.
The knock on the inherent INsecurity of Windows in general is probably why Vista has been such a turd on arrival. They overcompensated on the security aspect and adversely affected the overall performance of the OS. That and redesigning the entire interface (if you use the 2007 version of Office, you get a taste of that via the "Ribbon", which many people have learned to hate).
I'm holding out hope that Windows 7 will be somewhat decent. If not, I'll go back to trying my hand at Linux.