Tried to revert to XP, now my MSI 1029 notebook won't pass BIOS!

I installed Vista RC1 and after messing it up so that no sound driver would
install I decided to switch back to XP. So I booted off the XP CD and for
some reason it decided to start copying files to hard disk straight away
without giving me the option to reformat. I remembered I had this issue
before with Vista Beta 2 but couldn't remember how I fixed it, so I powered
off and powered back on and the same thing happened. I powered off again and
now the oddest thing is happening:

1. The notebook will not pass POST
2. I cannot enter the BIOS
3. I can however switch early enough to view POST messages, the last POST
message to be displayed is 'Ultra-DMA Mode 5, S.M.A.R.T. Enabled and Status
OK' and the system simply hangs, however if I press delete at this point to
enter the BIOS a message changes further up to say 'Entering SETUP...' but it
still stays there (so the notebook isn't completely unresponsive for some
reason it just won't go any further?!)
4. All fans work, if I insert a CD it will spin up
5. I am using BIOS version 2.70 (the latest one)
6. I can fill up the keyboard buffer and so it will start beeping when I
press more keys
7. This happens with CD in, CD out, battery in, battery out
8. Removing all sources of power for seven hours does not fix the problem

I'm probably listing a lot of junk but with this really odd situation I
thought no information is too much information! Any ideas?!