depending on what you need the XP system to do why not run it as a Virtual machine.
It could probably run everything you require. I wouldn't suggest using XP Mode but a full blown VM machine run under the free vmware product vmplayer. Download from the vmware site.
You can create the XP vm very simply from your running XP machine by using the vmware converter (also free). You don't have to worry about the hardware either - the Virtual BIOS takes care of that. The only hardware drivers
you need are for scanners / usb printers etc which normally run just fine on the VM.
A BIG problem for running a REAL XP on modern MB's is that most disks these days are SATA or SCSI so the initial XP system will probably come up with a message "No Hard Disks found" when you try and install the XP system. You will either need an IDE drive (slow disks BTW are a KILLER in ANY SYSTEM and usually present more of a problem than even a small amount of RAM or an older CPU) or a "floppy image" of the SATA drivers - and then since the XP install will only read external supplied drivers from a REAL floppy you are "screwed".
You can "Slipstream" the SATA drivers in to the XP system before installing it using nLite but ask yourself if its worth the hassle of doing this when a simple XP Virtual Machine will do all you want anyway.
I don't reccommend using XP mode since you have to have Windows 7 PROFESSIONAL at least -- vmware vmplayer will even work on Windows 7 basic edition !!!!.
Another advantage with the vm is that you only need to have it running when you want to run an application on it -- you don't need to re-boot the host machine.
You can also "Clone" the VM very simply if you want to test things.