|Are they setup up as a workgroup or as a domain, how did you setup it up??|
If they are setup a a domain you need to add the domain name and DNS IP to the XP PC, and set up the 2000 pc as a domain controller??
If setup up as a workgroup then you need to run the 'setup a small or home office network' wizard on both PCs giving both PCs the same workgroup name.