Despite my dislike of M$ I have to admit that XP Pro on my kid's machine is nice. Almost as nice as Suse Linux. Sorta wish I'd have gotten 2000 Pro though.
If Win98 works for you and you have no pressing reason to change then DON"T. I run 98SE on my main machine and it's stable enough.
If it ain't broke then don't fix it.
Even with multiple accounts you can set XP to autologin to whatever account you like at bootup. Can't recall the details but it wasn't complicated. Not very secure though.