Further to my last post, I have made some headway. After a 15 minute wait on the Microsoft tech help line, and a fruitless 15 min conversation, I decided to try uninstalling everything except the system: which I did (good thing I hadn't got too far my install in the first place...). Anyway, after I uninstalled the video driver, the problem went away - isn't that interesting? I'm now going to spend time trying different versions of the nVidia drivers to see which one will work with my 6600GT card. I will post results.

P.S. The motherboard is a Foxconn NF4UK8AA - I downloaded the nForce 6.33 drivers from the Foxconn website and used those for everything except video, in place of the 6.29 drivers that came on the motherboard install CD.