os x mountain lion 10.8

Welcome to the Apple Support Communities

Have you got any problem with Mountain Lion? If not, insert the OS X DVD that came with the Mac, press C key on boot, go to Utilities > Disk Utility, erase the drive and reinstall Mac OS X. If your Mac didn't come with DVDs, press Command, Option and R keys while your computer is starting, open Disk Utility, erase the drive and reinstall OS X

By erasing the drive.

Why?

P_20_J wrote:

Thx for reaction. This was what i need.

You do have a backup of some kind I hope?

You're welcome