it will be very slow yes, but if you have the existing equipment, not totally worthless. Just don't expect performance. Basically, you'll have on PC with a modem and NIC and another PC with a NIC... connect these two PCs together with a crossover cable. Give them similar IP addresses. In your network settings, find your dialup connection and share it. I've done this with windows 2000 and it's possible with XP too. Be persistent, keep an open mind, and be ready to dump a little time into it... there's always an obscure network setting that can be missed on the first time.
Oh, and you will make the gateway on the second computer to match the IP of the PC with the modem
hope that helps