Trouble connecting XP to LAN and T1 line

We have an established network in our office that uses static IP addresses with a subnet mask of 255.255.255.0. We recently installed a T1 line where the ISP provided us with a certain set if IP addresses to use the T1. The T1 also has a differentsubnet of 255.255.255.248.

I can have my settings such that I either connect to the LAN or to the T1 but not both at the same time.

I am running Windows XP Professional on a Compaq Presario 700. Does anyone have any suggestions?