Here is a step by step way to get a dual bot with 2 hard drives having Win XP one the first and Win 7 on the second. You may need to reinstall Win 7 if you unhooked the first hard drive when installing Win 7. By installing it the way you did, the computer will see 2 C drives instead of a separate C and D drive.
1. Make sure both hard drives are hooked up.
2. Boot from the Win 7 installer disc.
3. Click on "Install Now"
4. Select the Hard Drive you want Win 7 on. If that disc is already has Win 7 on it, go to disc options and reformat it.
5. Finish the Win 7 installation.
6. Reboot and select your OS.