OK, this is what I would do, if I wanted to start clean:
AFAIK = As Far As I Know, see http://www.gaarde.org/acronyms/
I don't use very many of these, but many posters use a lot of them:).
1. Change the boot options in the BIOS so that the CD/DVD drive is the first option. Decide what sizes I want my two partitions for the two operating systems (XP and Vista) to be. I use 20GB as a standard size for the O/S and all programs installed (and the pagefile).
2. Make sure that only 1 hard drive is attached, the drive I intend to install Windows on. This is just a personal preference of mine
3. Insert the XP CD, boot or reboot the system.
4. CD should boot. Follow screens indicating that I want to install XP, not repair.
5. After the XP CD scans the system and reflects installed partitions on the hard drive...delete any/all existing partitions.
6. Entire drive should now be reflected as unallocated, unformatted space. I create my first partition, as large as I previously have decided. I format it in NTFS, using the slow method.
7. After it appears that all files have been installed for XP, I can either immediately create the partition for Vista...or I can wait. I prefer to wait and see if something unforeseen happens to my attempted install.
8. I boot into XP (after removing the XP CD) and probably install a firewall (I prefer a 3d-party to the Windows firewall), a reliable AV program, and immediately (right after I install my firewall) go to the WinUpdate site for my share of critical updates which assist me in my security efforts.
9. I worry about drivers and programs after that.
Once I'm satisfied that all is good with my XP install, I would then approach my Vista install in the same manner.