This is just something else you might of been able to do.
- uninstall windows 7
- attach both drive to the computer
- install windows 7 (there is an option to install to a different drive when you use the costume install option this will make it so your computer will dual boot)
- now when you boot your computer you should get the options
Older version of windows (this will be xp)
use your arrow keys to select and your computer will boot to whichever you have selected.
You can choose which you would like to boot to by default by going to where is shows you your computer rating in windows 7, then click on advanced system settings, then under system and recovery click settings and there will be a drop down menu which you chose which os you would like to boot to default and you can also pic how long your computer will wait until is boots if you don't choose an option.