XP probs accessing software over network

I am a Technical Support Eng for a software company. My knowledge of XP is minimal, and currently we have only a few customers running XP, as our software has not been 100% certified for XP PRO.
I have a customer that has installed our software on (2) XP PRO machines. They are sharing a database directory on just one machine. ( 1 user has to access database files over the network). Everytime that user tries to access files, or save files over the network, it can take from 2-20 minutes to access or save. Any idea what the problem may be? We do not have this sort of problem with Win 98, Win NT, Win ME, or Win 2000 Pro. I appreciate any help someone can give me. Thanks!