Do stop by the LBS. It'll probably be more economical to fix what you have, even if they do it for you, than buying a new bike.

And without knowing what model of bike you have, it sounds like a solid foundation to build a commuter on. Plenty of us prefer the older, rigid mtbs with slick tires. The one in my avatar is an '06 Kona Smoke, technically a hybrid, but is akin to an older rigid mtb. I have 26"x1.5" tires on it- with fenders.