Vista - network bridge problems

Here's my network configuration:

University LAN <---> Fiber NIC <---> VistaPC <---> Ethernet NIC <---> Switch <---> Other computers in my office

For about a year I've been using this setup with the Fiber and Ethernet Nic connections bridged so that the other machines in my office could access the Internet.

Now suddenly today, it's stopped working. In bridged mode, the other machines in my office can connect to the network, buy my VistaPC can't. If I remove the bridge, my PC connects fine, but then of course, the other machines can't connect to the internet.

Using the "diagnose" tool in networks and sharing center says something about "multiple networks" but there doesn't seem to be a way to remove either of them.

Any ideas?