I have a Windows XP Virtual PC installed, which I have modified slightly. How do I install another copy to run in parallel?
You can create a second instance of the XP mode virtual machine by basing it on the same base image as that of the instance created by the installer.
There's a good blog post here that walks through the process (in this case, the author is creating the second VM in order to run IE 6, 7 and 8 side-by-side).
EDIT: I should also point out that this process can be used as many times as you like, so you can create more than two copies should you wish to.