You may have a bad HD there. Try to get it to register in BIOS by replugging, swapping cables, checking all connections. When it registers, download and burn to CD the maker's diagnostics/repair CD to run a full scan: http://www.tacktech.com/display.cfm?ttid=287
Then run Disk Check
on the XP HD either from Windows 7 or booted Windows 7 DVD command line.
When you tap the F11 key at bootup: do you get the BIOS Boot Screen menu and can make the choice to boot either the Windows 7 HD or the XP HD, but when you choose the XP HD it boots to Windows 7? Is this correct?
If this is the case, and you want to use the BIOS Boot menu to access your Dual Boot, then temporarily unplug the Windows 7 HD, set XP first HD to boot in BIOS setup (after CD drive), then run an XP Repair Install following these steps: Repair Windows XP - How to Perform a Repair Installation of Windows XP - Part 1 of 2
Once XP starts up, plug Windows 7 HD back in, set the preferred HD first to boot in BIOS setup, then boot the other HD when needed by tapping the F11 key at bootup.
Feel free to ask back any questions to help you perform these troubleshooting steps.