Freezing when disconnecting with DSL

I recently installed a NIC and a ADSL modem. It connects and stays connected with no problems, but when I exit or disconnect my computer freezes. ALT,CTL,DEL tellm that OLDWinAp is not responding. My computer is a Dell with a Celeron 450, 64 mbs RAM, OS is Windows SE, 3COM NIC and a Westell ASDL Modem. I tried removing and reinstalling NIC and Modem. I also connected a 486 with a 3COM NIC to the same modem (at different times the computers are not networked) and it disconnects without any problems. Does anyone know how I can resolve this?