Why is XP slower accessing the network?

We are slowly upgrading our computers. Buying much faster machines, with Windows XP. We have a Win2K server. The users with XP are complaining that it is slower. When they open and/or save files in the shared documents folder on the server, it takes several seconds to open, and several seconds to save. With Win95/98, it was almost instantaneous. The machines are set to use the Server for DNS. Any idea why the access is so much slower? Any advice would be appreciated, since they’re getting ready to lynch me for recommending this “upgrade” that has slowed them down!