Why does the cool air coming out of my window unit a/c smell SO bad?

You need to change the filter completely because it's likely packed with mold and bacteria, which can make it smell bad. Spend the few bucks and get a universal filter, preferably one that's anti-microbial so you don't have the same problem.

Frigidaire has a universal filter that's inexpensive and antimicrobial:

http://frigidaire.stores.yahoo.net/unancufi.html

It sounds as though you have a serious mould problem within the unit. If you feel comfortable with partially disassembling the unit, you'll likely find the stuff growing on the heat exchanger tubing and fins. Clean it with amonia and a stiff bristle brush, rinse thoroughly twice with clear water, reassemble and you'll be fine. There's really no easy way to deal with the mould except going after it directly because anything you spray into the air intake that might kill the mould will also seriously degrade the metal of your unit if it can't be rinsed off right away.

Cleaning the back of the ac should get rid of the odor. Taking the back off and looking inside might show some mildew or something. Also the filter.