If they really needed the computer for evidence they would NOT be telling you they wanted to see your computer - they would show up with a search warrant. If they don't have anything, they might be fishing. i agree with Aaron, contact an attorney.
Also, if they do serve a search warrant, unless the officer writing it is a complete moron, the warrant will not limit the search solely to chat logs. They will be looking for anything related to such a conversation to include files, e-mails, digital images, file caches, etc.
- carl

