Welcome (I guess I've been here long enough to be friendly that way
My first attempt would be to boot up the Windows 7 DVD, and us that to completely delete all the partitions on the drive.
Then, re-boot the machine with the WinXP CD. That should, in the course of the install, find the drive and pretty much force you to create a partition into which WinXP can be installed.
As for the driver
issue - a lot of Win Vista drivers
work. But, I find it strange that Build 7100 didn't have all the drivers you need. It did for me, after I updated the MBO firmware and my board was, literally, only days old when I did my first Windows 7 install.
Edit: Have you attempted to do a Windows Update? I've downloaded three or four updated drivers for my machine that way ---- and they worked