Vista - Update Hang

Trying to update from Beta2 to RC1, using the 32-bit DVD supplied by MS.

The update appears to proceed normally, until it gets to the Completing
Update stage. Then it just hangs. After a couple of hours, there is very
sporadic HD activity. I still have the recurring three dots after the
"Completing Update" message, but nothing else.

I know I could try a clean install, but I'm puzzled why the update route
doesn't work. And if it doesn't work for me, what's going to happen to people
buying the released version?

System is a Dell Inspiron 9100 with loads of free disk space and plenty of
memory.

Any thoughts?

Thanks.