I'm 99% sure that you're actually seeing a UAC problem. There are multiple known scenarios where Windows Vista or 7 (and presumably 8) will allow the creation of a new file, but silently blocks modification, and returns a "file locked" error message if you try.
Try temporarily disabling UAC (remember, you have to reboot for it to take effect) and running the updater with UAC disabled. If that fails, go a step further and make sure you launch the updater from a command prompt with Administrative privileges.
I think the root cause has something to do with the installer spawning another process/thread/app that doesn't inherit the parent's admin rights for some reason, and does it in a way/context that causes Windows to silently block the action instead of presenting you with a privilege elevation prompt.