1997 Honda Accord Emissions Control Systems

I just bought this car in Jan 2009 from the owner of the car (as in not a dealer). I got the car checked up by my mechanic when I first got it and had several things fixed up on it. Since this, however, it seems as if my car has constantly been in the shop. I got a new catalytic converter (sp?), Had my transmission rebuilt when it went out on me, and had my A/C fixed. Anyway, today the "Malfunction Indicator Lamp" came on. It says in the manuel that it "indicates one of the engine's emission control systems might have a problem". So, my qiuestion is- is this the same as the catalytic converter that I should have already had fixed or is this something new? And. What should I do? How much should it cost me to get this one fixed? Thank YOu!