Changing logged in user on the fly

What I essentially want to do is progmatically change the user credentials in windows XP. I will be logged in with one account and I want to make windows think another user is logged in. I will have a smart card with both users details on.

This is because I have to use one account to logon to the machine locally and then use another account to bring up a GPRS connection. It is this second user account that I need to use for authentication over the network but windows is handing over the credentials of the first user as this account logged into windows.

Any ideas gratefully received.

Jimmy.