HELP!!!! Smokin A/C

I recently bought a used 2004 Kia Optima, and have been noticing some smoke comming through the vents sometimes when I turn on my a/c. It doesnt smell like anything burning, but is is concerning. It also has been having a foul odor when I first turn it on. Now it doesnt "smoke" EVERY time I turn it on, but it odes sometimes. Any suggestions as to what the problem is, and/or what I should do??