I just updated XP with Service Pack 3. That's a service pack released about a year after MS stated that it would stop supporting XP.
That's an indirect knock against Vista. Here is a direct knock: A colleague installed Service Pack 1 for Vista. During the next startup, Vista complained that it had corrupt system files. We tried a system restore, which required us to run check disk, which wouldn't run because the disk was in use, so we restarted and it it couldn't check the disk, because the system files were corrupt. And this happens automatically on Vista.
No wonder I and thousands like me were willing to pay MORE to AVOID upgrading to Vista on our new machines.