If you're getting error 100 on the App Store while trying to download Mountain Lion, try this:

1. Quit the App Store app.

2. Delete any old "Install OS X Mountain Lion" installers. You should delete them from _all_ drives (or unmount any other drives you have copies on). You must actually delete them, not just move them to the Trash (i.e. Move to Trash, then Empty Trash).

3. Delete ~/Library/Caches/com.apple.appstore (it's a folder; delete the whole thing).

4. Launch the App Store app, click to download Mountain Lion, log in when prompted.