Please post back a screenshot of your full Disk Mgmt drive map and listings, using Snipping Tool in Start Menu, attach file using paper clip in reply box.
We need to see your current config to advise you with certainty.
There are two ways to dual boot when you have separate HD's. The cleanest is to boot via BIOS: set preferred HD to boot first, then trigger the other HD when you want to boot it by tapping F-key given on first bootup screen for Boot Menu. This leaves the HD's independent to come and go as you please.
A Windows-managed Dual Boot makes the HD's interlinked and harder to remove one later. If you want to use this method, install EasyBCD 2.0 beta
in Win7 after doing quick forum registration to use beta. Add XP on Add/Remove tab, accept offered boot files, let it autocomplete, Save, Restart.
However if there is any anomaly with your Disk Mgmt configuration, it may require further changes so post up the screenshot.