Just built a new PC specifically to use for evaluating Windows 7. Problem was, the install failed about two thirds of the way through, shortly after rebooting and starting services. I tried installing on a clean machine and also over Vista - still no luck - and no useful error messages either!
After some internet research, I tried unplugging the DVI lead from the on-board Nvidea 7100 graphics port and connecting the monitor via a VGA lead instead.
This time, the install worked perfectly - and going on to 'Windows Update' after the installation was complete, almost the first thing it downloaded was new graphics drivers...
(It now works fine with the DVI connector reattached)
Hope this is some help to anyone with a similar problem. I now like Windows 7!