Upgrading to Windows 7 Hangs.

So I finally decided to go ahead and upgrade my main Vista 64 installation to Windows 7. I would have done it earlier, only I was waiting in hope for A.L Tech to release a firmware upgrade for the media player to see Windows 7 shared drives. That's clearly not going to happen.
Anyway, so I punched in the upgrade, and it went really smoothly right up until it slides gently into a kind of hanging situation. The screen is happily updating alternating between "Please wait..." and "Updating your drivers ...", the drive light flickers occasionally, you can move the mouse cursor, but after four hours of this, you just know it isn't going to go anywhere. So I hit the reset button. Click-click.
Magically, and happily, it automatically recovers and reverted to my Vista install. Whew.
I've tried this twice now, and I see some signs of this happening all across the blogosphere.
That is, unless of course, it really does want to take 11 hours to complete...
It looks like I'll have to do a clean install. And it all worked perfectly in my various VM tests.
Fisking Super.
[Update - it worked after 12 hours!]