Pros XP Pro SP2: (just MO here)
XP Pro SP2 has a lot of features the average user really needs. And they might even like as well. The security center does a decent job of holding the average users hand and forcing them to do some smart stuff...like run a firewall, nag them about not having antivirus, force them to turn windows update on, etc. If you're a more advanced user, you will find that annoying.
Fast boot times.
Has more drivers included out of the box.
Has system restore.
Cons:
Runs slower out of the box, needs more ram. Has a lot of junk that needs to be turned off to get it to run a little better.
Removed support for non-PNP hardware
More compatibility problems with older apps (some will argue, including MS...but I stand behind this.)
Has System restore.
Win2K SP4:
No extra crap on top (good thing for me anyway)
Runs better on older hardware, needs less ram.
Doesn't ask you if you're a retard or not everytime you open a folder.
Non-PNP hardware support.
Cons:
MS seems to be trying to force people to upgrade from it. I believe they've dumped support for future IE releases on it. Seems like they're putting updates on the backburner as apposed to XP.
A bit more complicated for average user, it doesn't hold your hand as much as XP.
Slower boot times.