I had a similar problem with Win2k/Pro. I found the answer in a How-To somewhere, but as far as I recall it had to do with the local security settings on the Win Box. Unfortunately my Win2k is in Danish with translated settings, but look for something that tells the pc to use encrypted communications with the server and disable that or those settings.
When I installed a Win2k/Pro box from scratch I didn't have that problem as the pc was "born" as a member of the domain and automatically disabled the setting.
If this doesn't work, try creating the exact same account on the XP box. It worked for me before I read the real solution.
Cheers,
Jens