What version of Parallels are your running? I'm presuming, since you bought it at a store, that it's a version prior to Build 1970. Versions prior to that build are not compatible with your Core 2 Duo based Macs.
In Parallels, use the Check for Updates command in the Help menu to get the latest stable version (1970). Install it, reboot, and try again.
A Parallels license is valid for only one computer. If you want to install it on two machines, you'll need to purchase a second license. (You can do that online as all you need is the key.)
As to XP Pro, I believe the retail edition allows installation on two machines. The OEM package, however, can only be installed on one machine. But, take my advice here with a large grain of salt as I use the MSDN editions exclusively.
Last edited: Jan 18, 2007