Ok, I did a little research. I found that in windows vista, boot.ini is no longer the reader. Instead, BCDEDIT.exe is. You can go to a command prompt and go to X:\windows\system32\bcdedit.exe /? to find out the commands that are available. There is a downloadable gui that allows you to modify bcdedit.exe (
http://www.pro-networks.org/vistabootpro/intro.php).
A quick and easy way to blow away Vista would be to re-write the boot sector for the Windows XP partition and then re-write the MBR.
1. Do a system backup
2. Use the Windows XP CD and go into the recovery console (make sure you set your cd to boot 1st in your bios and then boot from it and follow the directions to get to the recovery console)
3. Go to your system partition (C:\ is where XP should be)
4. Type fixboot and hit enter. This re-writes the system partition which contains the files that are needed to boot. In essence, what this does it re-writes the bootstrap loader program. In short, it'll start using the Windows XP process again with the boot.ini and all that other good stuff. Make sure you're in C:\ when you do this. If you're not in C:\, you can just do fixboot c:\.
5. Type fixmbr and hit enter. This re-wrties the Master Boot Record so that it'll start using the new bootstrap loader program that you just re-wrote.
Now make sure you can get back into Windows XP just fine. Once you do, you can format your Vista partition.