I ran into this one earlier today. An update gets “stuck” and refuses to install, giving this cryptic error message. It’s happened to me before, although rarely enough that I can’t remember the specific steps of what to do.
Here’s the fix:
If you receive Windows Update error 80070003 while checking for updates, you need to remove the temporary files that Windows uses to identify updates for your computer. To remove the temporary files, stop the Windows Update service, delete the temporary update files, restart the Windows Update service, and then try to check for Windows updates again.
[Detailed steps follow]
Took just a few seconds, and things started working again.
So I share this with you in case you ever need it.
Ed Bott (@ wordpress.com) 
Windows update troubleshooter is built into win7 and 8, just a start search away.
I’ve run into this many times. I really hate this error, but it is quite easy to fix. There’s also a Microsoft Fixit routine that fixes it quickly too, if not easier.