If you are in this situation then you can get yourself out using these instructions.
If you used any installer other than msiwindosx86 you'll need to do the following before you run the 10.5.5 updater
while sleep 1 ; do rm -rf /System/Library/Extensions/AppleIntelCPUPowerManagement.kext ; done
handily msiwindosx86 uses disabler.kext to prevent the installation of the problematic kext.
After I updated, the machine started and then restarted and then started up OK. I then installed the updated kexts from rapidshare.com/files/156863271/MSIWindDrivers.mpkg.zip.
Another restart and things seem OK apart from the screen resolution which is wrong, and then I get a pop up that says some of the graphics drivers have not been installed correctly. So I re-installed AppleIntelGMA950.kext and AppleIntelIntegratedFramebuffer.kext using Kext Helper b7.
Another restart and the screen resolution is OK.