Mark Windows 7 active in Disk Management.
Unplug your XP HD. Make sure Windows 7 HD is first HD to boot in BIOS, after DVD drive.
Boot into Windows 7 DVD Repair console, click through to recovery tools to run Startup Repair 3 separate times with reboots. This rewrites the MBR to Windows 7 HD.
Now you can plug the XP back in, remove Windows 7 from dual boot in msconfig>Boot tab.
Then boot either one from the BIOS shortcut key for Boot Menu, set Windows 7 HD to boot first in BIOS. Drives are now independent and can come and go as you please.
If you will post back a screenshot of your full Disk Management drive map first, we can advise you more fully. http://www.sevenforums.com/tutorials...en-forums.html