I'm posting here cos I've gotten this problem in XP and Windows 7 - And it is the same cause and same fix.
Delete ALL of your Dot Net 4 installations, this includes "Extended" and "Client Profile" - It will want you ro reboot, but don't do it, try WU again after you uninstall these, it should work.
I ran across this in Windows XP, and that's what was the root cause of it.
But I could not understand, why I just did an "In Place" install of Windows 7 on a machine that was getting updates - I had to do it because my Boot sector had gotten messed up, after the "upgrade" I noticed that I was getting a message "Dot Net 4 needs to be repaired" - It repaired then I rebooted, and NO updates after reboot.
So I just thought, what if the same thing causes this error in Windows 7 as in XP, and Bingo, it was.
Take out ALL dot net 4, do not reboot, then try WU again, if it works, update all including dot net fixes - Then DOWNLOAD the stand alone Dot Net 4 client profile. Don't install it until you've cleaned your system out with ccleaner and Yamicksoft Windows Manager using "1-click clean" - Then try installing dot net 4 again, if it still updates after that, all good, but if not, then delete the stand alone dot net 4.
I don't know why dot net 4 causes this, but it does. it's error number
I don't know if the error number is the same in XP, but in XP removing the client profile fixed it.
OK I see 'RinceWindz" came to the same conclusion, but just for identification's sake, its the Dot Net 4 so-called 'Fix"