Nothing wrong with the mid 80's Schwinn mountain bikes that a diet wouldn't cure.

My current commuter is a Schwinn High Sierra (vintage 1985 or so). I bought it off a guy back in school for $100 and it still rides as good now as it did then. Of course, I've replaced some of the parts but mostly it's stock. I've put slicks on it, headlight/rear blinkies, clipless pedals, a set of fenders and a bike computer. My commute is just over 10 miles one way, and the only time I regret the bike is on two climbs I have on the way home.

I'm not sure what all differences there are between the Sierra and the High Seirra. Does your bike have the heavy welded up front bars? On the High Sierra, the front bars are one piece, all welded steel.