You need to restore OS X. No need to reformat the drive.
Help for restoring v10.6 here > Mac OS X v10.6 Snow Leopard: How to Erase and Install
If at some point you want to upgrade to Mountain Lion, make sure your Mac meets the requirements > System requirements for OS X Mountain Lion
Instructions for upgrading directly from Snow Leopard to Mountain Lion > Apple - Upgrade your Mac to OS X Mountain Lion