The culprit? A beta version of the Cisco VPN client. However, removing the offending application isn't as easy as just using Add/Remove programs. I had to go through a series of processes. If you're having problems connecting (and from reading tons of message boards, I know some of you are having problems). So, here's what we did to solve my problem.
Second, I fired up my BIOS and disabled the second onboard NIC, so that Windows wouldn't even know it's there. That didn't work either, but it did make me feel like I was doing something useful, which is really all I was looking for by this time.
Finally, I uninstalled the Cisco VPN software, and manually removed the offending part of the app, the Deterministic Networks driver using these instructions . After a reboot, I was in business, and able to log into Live and play Shadowrun and Halo 2 online at home.