Since you do not fully explain where on what point the update fails to work i'll have a short guess here:
Recently Win 7 did have some updates. Net4 was 1 of them. Probably you have your settings for these updates set to "automatic" and so your OS will download and install updates accordingly.
It is possible that, for some reason unknown to me, some updates are in conflict with each other and as a result, installation will fail.
Shutting down you PC will give you the message: updating, don't turn of your machine and then shuts down as intended by it's own. After reboot, you will get the message installing 0% and after some minutes this process will fail, reverse the update and you are back to the start.
Possible sollution is to install this update by hand.
Therefor go to Start->all programm's->Windows Update. There you can open the list of current updates, deslect them all and then select 1, update etc.
Most likely (if this case is indeed your case) your problem is solved.
If im beside the point please explain more in detail in what phase your update fails and what error message you get.