problem fixed for me

I had exactly the same problem. After I read this thread, I realized that
I am not the only one around the world.
I spent sometime today, and finally fixed the problem in a non-technical way.
I think the problem caused by laest update for windows vista media center,
when you install this updates .net optimazor got stopped, and then series
error message appears, then the update fails. If you restart from this point,
you will have this problem.

Soltuion I used:
Boot from DVD, then choose repair windows. after a few screen, you got to a
menu, choose start commnd prompt.
under command prompt, go to your anti-virues software folder(in ny case,
avast), renname all the exe file to something else, I run ren *.exe *.fuk.

restart the PC, boot fromt he DVD. repair windows. This time the DVD will
ask you if you want to restore to the original point, answer YES!

After a few minutes, the process will be done. restart the system. And you
will back to busniess.

My system is DELL D531 + VIsta x64 Ult.(Chinese Simp.)

I am not sure this wil work for you, but it did work for me. Good luck to you!

Jiang