Lost Server – If Found Please …..

I recently migrated from a Novell server to a new Windows 2003 Server on a small network. All the hardware is unchanged, only the servers operating system, and the network clients have been changed. Now all the XP client PC will loose thier network connection to the server after a period of inactivity. If a PC is left running without any network activity for aperiod of several hours it will loose the connection to the server – relogging in restores everything.

Any suggestions as to the culprit