<p>I'd like to convert a Parallels Virtual Machine image on my mac into an image usable by Virtual PC 2007.  Does anyone know how to do that, or if it is possible?</p>

It looks like qemu-img from
qemu
can do this, at least looking at its commandline help on a Ubuntu 8.04 machine where it claims support for, among others, the "parallels" and the "vpc" format.
Have not tried myself, though. Hope this helps.