I just installed Win 7 (64) on my Mac Pro 2008. It's a pain but not impossible.
The bootcamp assistant in the utilities draw is broken, so it won't download the latest bootcamp drivers, but that can be sorted later.
On the first screen, choose "I have the Mac OS X disc that came with my mac..."
On the next screen you have a choice of either creating a windows partition, or start the windows installer.
I would pick the the first option and then choose the HD you want to install Widows on.
Then start the installer. However, it gets a bit tricky, and confusing, when you first boot into the windows installer, as windows doesn't recognise the drive, however another reboot, holding down the option key to boot from the windows installer disc, and it then works. You also have to make sure that when windows does it's first reboot (and subsequent), hold down the option key for the boot screen, and choose the windows HD rather than the windows disc otherwise it crashes out.
Once windows is installed (and running), pop in the DVD which came with your mac, I'm assuming tiger/leoaprd? Windows will recognise the disc and you can install the bootcamp drivers. Then go over to Apple and download all the bootcamp updates and do them one at a time, and I think you get up to version 3.3.
That worked for me but it wasn't as straight forward as I'd hoped.