Authenticating with Domain

Xref: kermit microsoft.public.windowsxp.network_web:196304

I am experiencing these problems on XP Pro machines but not on 2000
machines. I log on locally to the XP machine with the same login info as on
the domain. If I connect to the network using a network card there is no
problem, either locally or remotely using a VPN. When I connect using my
modem and VPN, instead of authenticating with the domain using my windows
login, it tries to authenticate with the login info I use to connect to my
ISP. How do I correct this behavior? This does not happen on 2000 machines.