Sounds like your Master Boot Record (MBR) is on your old HDD.
To fix that disconnect the old HDD, boot up with the Win7 install disk and choose Repair your computer. How to Repair Windows 7 Installation
To add WinXP back to your boot options download and install EasyBCD
In EasyBCD you would add a New Entry & choose the Windows Tab.
From there you would choose the NT/2K/XP
type and choose a name for boot menu choice
Check Automatically Detect Correct Drive and you should be able to use the Add Entry button.