It should work as a boot drive, but you'll have to do some extra work, such as an XP Repair Install, followed by installation of the chipset and other drivers, to enable it to boot. If you're really lucky and the machine boots, you can skip the Repair Install step.
If you're really ambitious, you can configure a multi-boot so that you can boot from either of your hard drives and access the applications installed thereon. Last weekend, I configured a homebuilt with two storage devices in just such a fashion: the primary boot device is an SSD with Windows 7 64-bit, and the secondary boot device is a 750gb notebook drive with XP Home and Windows 7 32-bit (the latter originally created for a different chipset).
