Three years ago, I was in the same situation as the OP--interested in cycling, but without the money for a bike-store bike. My dad had an old Roadmaster--originally from Wal-Mart--sitting around in his garage, gathering dust. He gave it to me, and I took it home, tinkered with it, fixed a couple of minor problems, and started riding it every day.

After thousands of miles, nothing had broken. I did have to replace the tires (which were in bad shape after sitting for years), but it provided me with good service for a long time.

Last summer, I bought new wheels, and last week I converted it to a single-speed, so I'm no longer using a lot of the original components (derailleurs, shifters, etc.) I also bought a new seat, just as a personal preference. But almost none of the original stuff wore out.

In the meantime, I've bought a couple of nicer bikes, including a recumbent and a Trek hybrid. But I still ride the old Roadmaster. It's not the finest machine out there, but it's worked fine for me.