After I upgraded a Xen server, a Windows Server 2003 guest lost it's network connection. Apparently, the virtual NIC hardware had changed, so a new, unconfigured network device was installed.

(The virtual hardware had changed enough that Windows required reactivation. However, I couldn't do the online activation, because the new network adapter wasn't configured. I was locked out of the system. I had to call Microsoft's activation hotline and read nine six-digit numbers to a computer on the other end to activate Windows again.)

When I tried to configure the new NIC with the system's static IP, I got a warning since the old NIC was still using it. However, the old NIC wasn't in the Network Connections list or the Device Manager's Network adapters list.

I was finally able to delete the old NIC using the fix in KB269155.