Unplug your XP drive
Install 7 to your new one - Patch it, etc etc etc.
When you're done, shut down and plug your XP drive back in.
From here, you have two choices: You can press the F8 (sometimes F8) key to choose which drive you want to boot from. Or, you can boot to your Win 7 DVD, and do an installation repair - That will add XP to Win 7's boot manager. How to Dual Boot Win 7 and XP