Hi there
Due to the Boot mechanism differences between XP and Windows 7 in general you should install the OLDER OS FIRST.
What's happening here is that the XP boot mechanism which is different from Windows 7 is attempting to hose up the boost files.
Some people have got XP to dual boot with Windows 7 installed first however but if it doesn't work install XP first.
These forums have an excellent tutorial for BOTH CASES.
Dual Boot Installation with Windows 7 and XP
You can "Force" the XP system to work when Windows 7 is installed first by "hiding" the Windows 7 partition and setting the partition on which you want to install XP as "Active". GPARTED will do this.
However you'll then need to fiddle around with a boot manager.
My advice would be to wipe the discs, re-partition and start again installing XP first.
Windows 7 also often adds a small ACTIVE "System" partition which causes problems when installing XP as this is the "Active" partition so again I'd suggest you'll have a lot less problems installing XP first.
Don't forgget also with XP you probably will need to "slipstream" SATA
drivers or your XP system won't find any hard disks -- you might be able to set the disks to IDE in the bios which will get round the problem.
Cheers
jimbo