Figured it out. There are two ways, actually:
1) Add another NIC. Set one NIC up for DHCP, and the other one up with specific IPs (using the Advanced button on the TCP/IP properties dialog). Since this was for a VirtualPC image, it was a simple thing to tell it to add another (virtual) NIC.
2) Hack the registry. Here's an article I found (but did not test) that shows how:
Steven A Smith
President, AspAlliance LLC
CIO, Lake Quincy Media, LLC