A new hard drive?
If you only have the one drive, then a new hard drive (£25 or so off eBay for 250 Gig) is a good option. Has the advantage that you aren't going to screw up anything on your existing drive, & you can copy over all your precious-es at leisure... and if all does go pearshaped you can revert to W2000 - leave the old drive in the m/c.
I've used the two-drive route every time I've upgraded and it works.
As for MctheKnife & 'best operating system' - well, I never had the blue screen of death on XP pro - lots of times from 2000. But then I do mess my m/c around a lot. Horses for courses, I guess.
Good luck,
Jim