I’ve had Windows 7 on a few of our machines for quite some time. My wife’s machine got it right after the Beta ended (one of the free copies some Beta participants got), my test / dev machine got an MSDN copy the day it showed up on MSDN, etc. I upgraded my main notebook the first day it was available and I upgraded my son’s notebook yesterday. Then I tried to upgrade my daughter’s notebook. (Mine, my son’s, and my daughter’s are all Dell Latitude D820 machines with identical hardware configuration and Windows Vista Ultimate). Her machine went through the gathering files, expanding files, installing updates etc. just fine. It would get to the last phase where it moves the files back in place and would fail and then go into a reboot loop. It would say that the upgrade failed and it was restoring the previous version of Windows – then reboot in 10 seconds over and over and over and over – well you get the idea. It wouldn’t stop rebooting. This isn’t the “62% hang”, it isn’t the "BSOD” one, and doesn’t seem to have a solution yet.
Fortunately, I always make sure we have a current backup on Windows Home Server. We restored this (first time I have needed to) and it was right back to Vista running fine. Tried the upgrade again (after first making sure the upgrade advisor was green) and got the same problem. Restored from Home Server again and went to bed. This had taken the whole day.
Strange how both of our other D820 machines upgraded just fine.