As title really... I want to install Windows on a VM created in Virtual PC on my Windows7 PC, and be able to run it using Parallels or VirtualBox on my Mac.
One solution would be to create similar VM in VirtualBox and tell it to use the same virtual hard disk as Virtual PC - VBox has VHD support, so it should work well with VPC's hard disk file.
But there's a number of problems with that setup. First, VPC and VBox emulate different hardware. If the guest will be running Windows, it will fail to boot on the second VM. Linux distros should work, though.
Second major problem is that these two VMs won't be synced (until you're running both hosts on the same machine or the HDD image is on an USB drive you attach to one or another host).
I'd suggest to run that VM in VirtualBox both on Windows and OS X host. It will solve most problems, if not all except for the synchronization issue.