I'm running my laptop WinME, Server XP, and Game machine on 2000, all talk to each other quite nicely now, all sharing the internet
however had problems like what you are discussing.
Try running the network setup again on the xp, it will then hopefully re-discover the win2k.
Dont forget to share the drives.
I found 2000 initially made it hard to share the drives, found something funny about the c: drive on 2k not wanting to share, but ended up changing the share name somehow, and that fixed it.