repus, I will tell you what I told my own father in law last night - and it broke my heart to have to say it.
You should look into spending more money on a bike.
Not knowing anything about them Craigslist will be taking you for a ride more likely than not.
Wal Mart sells sculpted scrap metal that just sits in garages waiting to rust (too heavy, too unreliable to be used as real transportation).
I know "spend more" is never good advice, but think about the cost in terms of the fact your buying transportation and a gym membership that will last you through college. If your willing to ride in the rain, it sounds like your prepared to save hundreds of dollars against the cost of other forms of transportation down the road. Personally I would have taken out a a student loan to get a decent bike if I had known what my work/class situation would be like back in college. As it was, I was doing 13 mile commutes on a beater to get to my night job - you never know what will happen, but you do know that a bus pass or a car is a high fixed cost.
Last year, I know Felt had some good deals running for between 3-400 at various places online. Performance Bikes & Nashbar might help. If you have to go through second hand bikes....try another student who used the bike in the same way you will.