Repairing may work.
If its a 2k that was upgraded to WinXP... well nothing may work....

Upgrades tend to be a problem for everyone that I've encountered....
Trying to reinstall or repair things, or simply upgrade service packs usually give some sort of problems.

Repair
Reinstall Windows over current windows
Reinstall in a different partition/folder
Format/Reinstall

In that order if you'd like to try.... but back up things first.
I've found some versions of XP/2k didnt have repair options though.

At the moment, must have sleep though... if need more help I can round up some clearer instructions on how to do each. As Unne suggested, reinstalling(and/or) reformating windows is usually the QUICKEST and cheapest by far unfortunately.... Truely... Windows not meant to be understood... unlimited software compatibility = ultimate software failure....

Getting into CMOS settings via F1 is the standard, I've seen it vary on some proprietary systems though.... monitor doesn't come on at all?