NT or Win2k?
For the former there are several additional things you should do, but for Win2k my faq should resolve the issue: faq779-4017
This will sort the issue for Win2k. I would add two additional things:
1. Disable the Webclient service on the XP clients if not needed, and it rarely is;
2. SP4 for your Win2k DC and BDCs. There are several important patches as part of this Service Pack.
If this is NT please advise. The settings are completely different, but the faq listed above handles many major issues including autonegotiation issues (XP is much less tolerant than earlier OS versions), SMB signing, and asynchronous logon issues.