I believe what you have there is/was the Shimano FFS (front free wheel) system, which did not prove to be a success for the Big S. I'm afraid I have nearly zero experience with that product (and want to keep it that way) so can't offer you advice as to whether it can be saved, but if the BB shell has conventional threading (maybe) I'd replace it with a conventional BB and crankset. I'm sure somebody here (or in the mechanics forum) can tell you what your options are.
You can do an archive search here (as well as checking Sheldon Brown's site) for more info on the FFS.