Actually I'm not all that impressed with it. I've tried it on my LCDs and wound up turing it off. It makes the text that was razor sharp, kind of fuzzy.
If you want to see what it does on Win2K, open Acrobat Reader (assuming you have it installed) and go to 'preferences' - 'display' - click 'use CoolType' (that's what Adobe calls their ClearType!) and see for yourself if WinXp is really worth the troubles to get ClearType.