|You can actually install XP on the second hard drive, but since Windows 7 is already installed, you will need to then put in the Windows 7 install disk and do a start up repair. What happens is that XP overwrites the boot loader for Windows 7 so Windows 7 will no longer boot. When you do the start up repair, the Windows 7 boot loader is reinstalled, but upon seeing XP also installed, it will set up (or offer you the option) the dual boot for you.|
The other option is using your BIOS to select which system to boot to. To do this, you set both hard drives to being bootable in your BIOS, then shut down and unplug the hard drive that Windows 7 is installed on. Then you install XP on the second hard drive, shut down and plug in the original hard drive again. This way they each have their own boot loader, but it is up to the BIOS to select which drive to boot off of. You may need to go back into the BIOS set up and select which hard drive you want first in the boot order (just after the CD/DVD drive) for the 'default' operating system. Depending on your BIOS, you may be given a boot option, of you may need to tap F8 at start up to be given the additional boot options.
As mentioned, installing XP first would be the optimal method and the one preferred by most. If you were also installing a Linux distribution, you would install that last since it's boot loader can launch Windows OS's but not the other way around.
You have to be a little bit crazy to keep you from going insane.