i have about 40 3rd party softwares, don't most people?
Maybe, but most people don't have a major built-in component of the operating system failing on a Mac they "just got."
My thought was that possibly something you installed is conflicting with Preview. Since I'm not psychic, I have no idea what that might be...if it's even the case.
Are you certain that all forty of those third-party apps are 10.6-compatible?