To upgrade to Windows 8 I wanted to avoid a re-installation of all my apps. and data etc so I went with an in place upgrade. This all seemed to work properly and soon I was running Windows 8 and could access the Windows Store App templates from Visual Studio. However, soon after Windows 8 kept crashing, well freezing. It got to the point that after every reboot I'd be lucky to get 5 minutes of up time between each freeze.
Given that Apple haven't provided Windows 8 drivers yet this wasn't exactly a surprise. I decided to try and work around this by rebooting to OS X and using VMWare Fusion to access the Boot Camp partition. Whilst rebooting in OS X I managed to corrupt the Windows installation. I use a non-Apple wireless keyboard (as I need the insert, delete, home & end plus the easily accessible cursor keys for VS development) so holding down Alt to select the OS to boot into didn't work. When I realized it was going back into Windows I just turned the machine off. After a couple of times the Windows installation was toast! To get back to the point of trying Fusion I had to do a fresh Windows install. In this case installing a minimal Windows 7 installation: just enough to allow the download of Windows 8. I then installed Windows 8 using the preserve nothing option.
Having now gone through the steps I wanted to avoid I decided to give the new installation a go via direct boot, i.e. no Fusion. That was two weeks ago. Since then I've re-installed all the apps. and my personal data and (fingers crossed) haven't had a single crash. As the freezes were usually happening during some graphical operation e.g. a status bar updating I assumed the fault probably lay with the video drivers. I didn't install Boot Camp assistant and in particular the Windows 7 drivers from OS X disc. Well, I did install one. After a while I noticed I wasn't getting any sound even though all the audio drivers and hardware claimed they were happy. Eventually I installed by the Cirrus Logic driver which made the speakers work. I haven't gone anywhere near the NVIDIA drivers.
So, the whole point of this post is for those who run Windows via Boot Camp on early iMacs and want to run Windows 8 then perhaps a fresh install (or maybe uninstall the Boot Camp supplied drivers prior to upgrade) is probably the way to go.