Boot into Windows 7 DVD Repair console, see if it discovers an installation to repair, click through to recovery tools list to run Startup Repair up to 3 separate times to recover the MBR into Windows 7.
This will likely make Windows 7 booted independently, however XP should still hold its abiility to boot, so you can try using a BIOS managed Dual Boot by setting one HD as default first to boot in BIOS setup, then if you want to access other HD use the F-key given for Boot menu on BIOS boot screen.
If you want a Windows-managed Dual Boot, which makes the drives interdependent upon each other and harder to extricate, install EasyBCD 2.0 beta
in Windows 7 after doing quick registration to use beta, then add XP on its Add/Remove tab.
If there are any further problems, post back a screenshot of full Disk Mgmt drive map and EasyBCD listings using Snipping Tool in Start Menu, attach file using paper clip in reply box.