Remove the XP HD, swap its cable to Win7 HD or set Win7 HD as first to boot in BIOS setup (after DVD drive), then boot the Win7 DVD Repair console or Repair CD, click through to Recovery Tools list to run Startup Repair up to 3 separate times with reboots.
After Win7 boots on its own, you can replug XP. The HD's will now boot via BIOS, with your preferred HD set to boot first, or trigger the other HD to boot by tapping the Fkey given on first bootup screen for one-time Boot Menu.
If you still want to delete the XP HD, then carefully select DISK then PARTITION using DISKPART, verify twice you have the correct HD/partition, then use DELETE PARTITION OVERRIDE command. I would then format it Logical for data so it cannot interfere with Win7 HD.