You can most likely edit the boot.ini file on the Windows XP computer to get it to recognize the second Windows install. Directions here:http://support.microsoft.com/kb/289022
Another option is to go into BIOS and manually change the startup drive to the Windows 7. Most motherboards also let you press F12 or F8 during POST to get a boot screen which will then let you choose a one time startup drive. Hope this helps!