From the other Windows 2000 boxes the machine shows up in Network Neighbourhood, but when I try to see what is shared I get an error saying the machine is not accessible.
Conclusion - Windows XP does not integrate easily into an existing Windows network!
I installed Windows XP on the same day as you and everything worked first time and it connected to my (XP) network, saw all the shares and connected to Internet Connection Sharing with one simple wizard. I think XP is very good at networking with other XP machines but it's notoriously bad for administration on big networks with NT/2000/2003 running on them.
This may be because the NetBEUI protocol is NOT installed by default in Windows XP. Based on my experience, Windows XP defaults to sharing folders over TCP/IP. In fact, those shares are advertised over the internet as well!
In order to get Windows XP to communicate with my older Windows boxes, I had to install NetBEUI from the XP disc by following the below tutorial: http://www.homenethelp.com/web/howto/HomeNet-Win2kXP.asp
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Ashley T. Howes Ph.D.