If it doesn't use any features of 7 that didn't exist in 5, and doesn't rely on any behaviours that changed between those versions, you could 'save for previous version' repeatedly using older versions of LabVIEW until you reached version 5 (do you mean 5.0, or 5.1?). If you don't have access to any of the intermediate versions, you might be able to ask someone on the forum who has them to kindly do this for you, or one of the NI representatives might do it. Unfortunately I only have 5.0.1 at the moment, so I can't help you!
If the code is really simple, it's probably easier to print or save the diagram and recreate it from scratch in the older version!
