When you get ready to proceed and need help, let us know.
For now, there are several tutuorial about how to move XP to another computer. You basically have to use the install option, but when the original install is found, you select repair and not install. This is not the first repair option you are shown, but the one after you go to the install page.
There is a utility called EasyBCD 2.0 you might want to download to your Win 7 install and have it ready. You will need to join their forum to be able to download it. It isn't completely necessary, but makes it easier.
Once you get both the Win 7 and XP drives ready to go, hook them both up with the Win 7 drive being the primary drive, or first in the drive order in the bios. You can also do this by booting with just the Win 7 drive connected and then adding the XP drive later.
As you go through the tutorials, you will basically need to copy 3 XP files (ntldr, ntdetect.com and boot.ini) onto the System partition. This might be the System Reserved, or the actual Win 7 partition. Then the boot.ini file has to be modified. EasyBCD will do this for you, but problems can arise if you have the 100mb System Reserve partition and will have to assign it a letter.
But let us know if you run into problems.