XPpro forces dialup when accessing NW

XP Pro PC on small NW with W2k server.

Whenever the user accesses files & folders on the Server, it invokes dial up (on the central modem on the server).

Other than a trojan, (which I doubt, since it has done this since it was initially set up) what else might be prompting dialup in this circumstance? I’m running out of ideas.

Thanks