The best way to set up a Dual Boot on separate HD's is to boot via BIOS. This leaves the HD's independent to come and go as you please.
Try choosing the XP HD to boot now by tapping the one-time BIOS Boot Menu key given on first boot screen:
- Asus - F8
- HP/Compaq - Esc
- Sony - F2
- Acer - F2
- Gateway - F10
- eMachnes - F10
- Toshiba - F12
- Dell - F12
- IBM/Lenovo - the blue Thinkvantage button
If XP won't start then post back a screenshot of your full Disk Mgmt Drive map using Snipping Tool in Start Menu and we can advise you how to set up Dual Boot via BIOS, or Windows if you prefer.