recovering from a nasty situation

Hi all,

OK, I am @ my wits' end so this is a 'this is where I am at, and this is
where I want to be post.

OK, so I have a desktop [currently in pieces] that I want to get vista onto.

Where I am:

The machine in question had a nice install of XP SP2 on it, all was good.
Some time ago I installed an early beta of Vista on it and it was dual
booting fine.
I did nothing with the box for some time.
Now I come to it, latest beta on DVD in hand.
Heres the thing; the XP boot now appears somewhat FUBAR'ed, it just does not
work.
The Vista install has expired, it wants a product key, I have tried a few (I
have full MSDN subscriber) and I cannot give it one it likes.
I have been into the BIOS and set the 1st boot device to CD and rebooted
with the DVD in the tray. By selecting the 'Boot into command line' option
into the Vista install I have determined that the CD ROM device isnot
recognised, hence the boot optoin is ignored. I have got the correct drivers
for the device on a CD but have not done anything with it yet.

Here's where I weant to be:

I want to FDISK the MDR removing all instances of previous OS's
I then want to go ahead and install the new beta.

My question is, therefore, how on earth do I recover this situation?

Many thanks in advance,
DC