Modem Dialing Problem Win2k

[KidKasha]

I have 2 ISP when I dial one it connects stating" Regestering your Computer on Network" When I log off in same session the seconed will not connect. It dails up and states thet it iverifying username and password, but doesn't connect. It makes no difference which one I dail first. After this my close down has to be reset. What could this be??