I chose the wrong partition to install Windows. How do I reinstall Mountain Lion?
You can boot into Recovery Mode by following the instructions outlined in HT4718 and re-install OS X from there:


  OS X Lion or OS X Mountain Lion includes a built in set of utilities in the Recovery System. Restart your Mac and hold down the Command key and the R key (Command-R), and keep holding them until the Apple icon appears, indicating that your Mac is starting up. After the Recovery System is finished starting up, you should see a desktop with a OS X menu bar and a "Mac OS X Utilities" application window. Note: If you see a login window or your own desktop and icons, it is possible that you didn't hold Command-R early enough. Restart and try again.