micro is loosing it's desktop OS market.
since Win98 nothing serious "happened" there. WinME was a failure, Win2000 is not exactly desktop OS, WinXP has all these disadvantages.
So in certain places Win98SE is still pre-installed as default. It would be popular long time even it's 5yrs old.
Win2000 is slow indeed if you don't know to configure it properly for non-networked environment and install everything you can ... It has high requirements twice+ higher from official micro's.
WinXP needs improvement, SP would probably fix some probs, but new one would come. I think it is not the best thing to install.

You want to know about disadvantages check Windows XP thread by keefe.

I would recommend to go for W98 but learn about W2K too

good luck