HT1338 Error 103 App Store Update…: Apple Support Communities. Thank you Derek for the answer. This is something that I am sure that Apple will fix.
I had the same problem, which may arise from having multiple “stacked” updates available—Mountain Lion 10.8.1, iMovie, and iPhoto.
This resolved it for me:
1. Under Software Update, click “Show updates.”
2. In the update window, click “More …”, which shows all updates available (in my case 3), and lets you install them seprately;
3. Install the 10.8.1 one first, letting the computer restart;
4. Repeat the process to install the remaining ones.
After you install 10.8.1, you may be able to install the remaining ones together, but I did them separately. Each of them is a long download.