Check engine light problem?

Hello. I am asking this quesionts on behalf of my girlfriend. She drive a 98 Saturn SC2. Every once in a while the check engine light will come on for a few days, then turn off and come back on again. How would we go about finding out what the problem is?
Thanks
Chris

autozone and many other autopart store will check OBD-II codes for free, don't take their advance about what to replace, just post the codes here and we will tell you what really needs to be done.