This happened to me today ....

I have Windows Update (for win7 64-bit) set to automatic updating and usually it works perfectly OK.

However today it failed to install 4 security updates, complete with computer rest etc.

Three times.

Eventually I tried changing to manual install and installing the failing updates one at a time, with a reset between each one.

Voila - all updates installed first time, no problems. Can only assume there was some sort of conflict between 2 or more of them which prevented them from installing en masse.

(So if you have similar problems - give the solution a try