Can I connect a stand alone PC w/cable inet connection to a Win2003 ntwk?

I support a small organization that has a single Windows 2003 server on a larger organization’s windows network which is part of a large citywide local government wide area network. “They” handle all the networking access control and communication and are very restrictive in their policies. The organization had the need for a totally independent internet connection that was totally unencumbered by their restrictions so we set up a single stand alone PC with a cable internet connection direct from modem to PC and we were golden. HOWEVER, now several months later, they want that PC to be able to access files on the server. What would be the best way to set this up so that on occassion a user on that PC could login to the network?