When your original license expires you can run the hardware ID program on your new PC. There are two programs that come with BTB but I can't remember which one you need to generate the file. Go back to the BTB website and upload it like you did before.
CreateHardwareId.exe is the one you need to use. it creates a file named hardwareID.txt which you have to upload to Bobstrackbuilder.com. Within 48 hours you receive a licence file, HardwareId.v22.214.171.124, which you put in the root map of the BTB program.