Which version of OSX are you using? (sounds like an OS 9 problem).

I'm guessing Apple already ran you through Software Update, starting up holding the shift key (safe boot), repairing permissions (Disk Utility), and restarting regularly.

If so, open the Console (it's in the Utilities Folder), try LimeWire again, and watch for any error messages. If you post them here they should help clarify what is going wrong.

Cheers