Can't open DMG files...should have never upgraded my OS to mountain lion

Upgraded to OSX 10.8.2 and now I can't run any downloaded dmg files. Says "the Following disk images couldn't be opened". and then say's there "isn't enough disk space" I'm pretty sure it has something to do with apps that are not approved by the app store. How can I turn it off?

I posted this question before and thought the problem has been solved nbut i'm still having this problem.