To add to Terry's advice above, if you replaced XP's boot code on its own HDD with the W7 version, then this will require putting XP's boot code back. The easiest way to do this in your situation is to boot into XP from the W7 bootloader, and then (while in XP) disconnect the W7 HDD, start up EasyBCD 2.0 Beta
, go to Manage Bootloader, select the "Uninstall the Vista bootloader" option, and click the Write MBR button. Then you should be able to boot into XP with the W7 HDD still disconnected.
You should test it though first before formatting your W7 HDD, to avoid making things harder on yourself.