Just do a clean install, not an upgrade! Reason why an upgrade isn't possible could have a lot points! That could be from change in Windows itself and regarding your hardware or driver related. Check the log files for more information.
I did an in place upgrade and had it save what it could which in this case was just personal files. Had to reinstall programs. As pisthai said could be many things, but I do know an upgrade will work.
On sudden hardware etc, it would! On others, it wouldn't! That's the 'Game' with Pre-Release software!
I try to make upgrade and not clean install because i have programs and i want not reinstall but finally after that proplem i make fresh install and all is ok its first time who i have proplem with upgrade no other times only with this version 9901.