Currently Being ModeratedFeb 7, 2013 2:44 PM (in response to c.gallino)
What version of OS X and which iMac do you have? One thing I can recommend is clicking the "More Like This" link to your right, you may find the solution in there.
My guess is you are new to these forums, going forward please complete your profle so we know what iMac you have and which version of OS X is installed. Once you have that information please pose your question along with any other pertinent information.