"Boot Camp x64 is unsupported on this computer model"
"This installation requires elevated privileges. Launch the installer through setup.exe."
Run BootCamp64.msi from an elevated command prompt:
"This version of Boot Camp is not intended for this computer model."
So what gives? You need to pass the NOCHECK=1 option to the MSI.
I've seen guides recommending modification of the MSI files or other old information that did not work for me.
Open an elevated command prompt (click Start, type "cmd", then right-click and run as Administrator), go to the the BootCamp\Drivers\Apple directory, then run this:
msiexec /i BootCamp64.msi NOCHECK=1
I was able to install Boot Camp (and its drivers) under x64 Windows 7 on my 2007 Mac Pro (macpro2,1) this way.
Note that this is not required for all versions of Boot Camp. It was needed for version 4.0.4255, but not version 4.0.4033.