2005 Chevy Cobalt Car shakes after hitting 50MPH

The car used to shake when on the highway, but before inspection I put new tires and brakes on, and the shaking stopped. (In Feb 2009) I havent really put that many miles on the car since, and it started shaking again. You can hear the shaking in the stearing wheel when driving. I don't know what would cause it to start again after it seemed to be the tires that was the problem last time.