Freezing of keychain after start

Hi, two weeks ago I bought a new mac book pro 13", I bought it for my work, but mainly for my school. I need an internet, but in our school we have a network, wich needs some certificate. I downloaded this certificate from school sites. It was a cert. for previous version of OS X, 10.6. I tried to install it by double-click but after that, the keychain frozen and since that time, every time when I want to start the keychain, it freeze. Is there any possibility how can I reset the keychain? Thnak you for your help.