DISCLAIMER: Do this at your won risk, I'm not 100% confident there are no problems with this procedure.
I would duplicate that windows xp .pvm file, give it a different name, then in Parallels, File>Open the duplicate (it will say something about duplicate MAC addresses, allow it to update). Go thru every setting in Configure and adapt it to the target bootcamp partition you have in mind, starting with changing the OS, then finally in Hardware>Hard Disk 1 change source to reflect the correct bootcamp partition (pick the correct hard drive by maker and model).
Try it.
I've personally done this in the past and worked in a little experiment I made, although I only used it once. The reason I'm not 100% sure is that, first, apparently PDM wasn't actively developed with the idea of supporting more than one Bootcamp partition at the same time, secondly, or, because of that, I think (not sure either) Parallels can enumerate the disks differently on occasions, and from what I remember in the configuration Parallels doesn't record the disks unique id but this enumeration that is made at the start of the app, this might be only problematic with external drives.
Last edited: Nov 30, 2011