Urgent - Computer won't boot after install XP attempt

Ok,
(Just to note before I start, I was trying to do the BIOS update so that the motherboard would work with the new AM3 CPUs (it's an MSI K9A2 Motherboard) So that I could upgrade it without having to buy a new motherboard.)

- I had vista and windows 7 beta installed on seperate hard drives (60GB for vista ultimate64bit and 120GB for windows 7 ultimate 64bit beta)
- I went over the windows 7 beta with the new RC1 version of Windows 7.
(it was working fine at this point)

- to try and get the BIOS updater to work (only seems to work in XP for some reason), I shrunk my windows 7 volume to half capacity, and formatted the remaining space as a new partition
- I put in the disc in for windows XP and booted from it, started the install from the disc onto the new partition I created by shrinking the windows 7 volume.
- it copied the setup files to the new partition, finished that ok,

- when it came to boot the OS from the hard drive to finish setup, it came up with an error saying there was an error loading OS


- it won't stop doing this no matter what I try.



I've tried
- using both my vista and windows 7 discs to restore the boot settings
- reinstalling windows vista
- removing the hard drive containing windows 7 and the new xp partition from the system (so all that was remaining was the hard drive with vista on it)

At this point I'm willing to try pretty much anything, I've already lost the installations for Windows Vista and 7, I just want to get it to boot an OS.