Thank you for your post.
You can convert your FileMaker Files from a .fp5 format to the .fp7 format by opening your current files in a trial of FileMaker Pro 11. You can find the trial and more information on conversion in Knowledge Base Article #10571.
The most convenient way to convert is to place all of your current (.fp5) files into one folder. You can then drag that folder and drop it on FileMaker Pro 11's Application Icon to convert all of your files at once and prevent breaking any existing relationships. For more information on converting multiple files at once click here.
Keep in mind FileMaker Pro 7 through FileMaker Pro 11 are no longer available from FileMaker Inc. and FileMaker Pro 11's support is ending September 25th of 2015. This would be a great opportunity to move up to our latest FileMaker 14 products. For more information on FileMaker 14 Products click here.
Filemaker 5 is 16 years old now. Looks like you got a lot of mileage out of that!