Move all your documents onto another Harddrive.
Reformat and Reinstall OSX, make a new User Account set that to Standard.
Move Documents back into new Account.
Note it is impossible to know what they have changed and added to your system! You must wipe everything except documents. Don't forget you Email, Contacts, iPhoto and iTunes Libraries. These may be huge.
This is not straight froward you may like to get an AppleCare Service Centre to do it, our Tech charges $150, but YMMV.