If your computer can handle it, get XP... ME was/is widely considered the worst operating system Windows put out, in terms of buginess and crashing.
But if your computer is getting up in years, might be better to just wait to you buy a new one (because it will come with XP on it anyway).
I held out against getting XP for the longest time, but it is very stable and starts up quick when I turn the computer on.
But shit, as cisco says, maybe don't fix what ain't broken.