I am sorry that you are having problem installing Vista. Are you trying to install Vista on separate HD or on separate partition? Are you using Boot Camp?
If you were installing on separte HD, can you verify that separate HD is NTFS (Windows) or GPT (Apple) or GPT/MBR hybrid (Boot Camp) formated by checking it with Disk Managment (Administrative tools -> computer managment) from XP?
I think you might have GPT formated HD and somehow Vista is detecting GPT during installation even after it has been converted to GPT/MBR hybrid HD by Boot Camp. It is my understanding that Vista 32 will not install if it detects GPT unlike XP or Vista 64 (although XP will not boot up after installation). Try to reformat HD to NTFS from XP and try again from Boot Camp.
I am using Vista 32, XP 32, and OS X triple boot. I have installed Vista 32 and XP 32 directly onto separate HDs without using Boot Camp. I used this route as this allow Vista and XP to be installed on straight NTFS formated HD without having 200mb GPT partition Boot Camp insists on creating for GPT/MBR hybrid HD.
If you want to try installing Vista on clean NTFS formated HD without Boot Camp,
1. Create Boot Camp CD or have Leopard DVD ready
2. Place Vista DVD in computer and shut down
3. Remove all HDs from Mac Pro including OS X HD and place clean formated HD in bay 1
4. Force reboot from DVD drive by Command-Option-Shift-Delete to bypass internal harddrive on boot.
5. This will force Vista to be installed on HD (free of GPT trace) as if you are installing on regular PC.
6. Reboot once Vista is installed
7. Run Windows Update to update drives
8. Use Boot Camp CD or Leopard DVD to update missing drivers
9. Update to latest Intel S5000VXN board, NIC, and 5000X chipset drivers from Intel (
http://www.intel.com/support/motherboards/server/s5000xvn/sb/CS-025509.htm ). You should not have any missig driver warnings at this point under device manager.
10. Try out your Pro Tools3 PCIe cards. If your Pro Tools program is Vista compatible, it should run.
11. Decide if you want OS X/Vista dual boot or OS X/Vista/XP triple boot.
12. Shut down. Leave Vista HD in bay 1 and place back OS X HD and other HDs. If you are going to triple boot, your current XP HD will not boot as you will need to modify the Windows boot loader. If you are planning to triple boot, I suggest VistaBootPro program at
http://www.vistabootpro.org . Just follow the direction at that website for dual booting Vista/XP.
Good luck