paid product (in this case, antivirus) came with personal serial number in which you use it to register/activate your installed antivirus. usually when you register, you will be asked to provide your email and other necessary particular so that in case you lost or forgot your serial number, or in your case, you format and reinstall your OS, you can ask to retrieve it (the serial number).

however keep in mind that the serial number should not be share with any of your friends or families (i mean using the serial number to install it to multiple computer that are not yours), unless you're buying multiple licenses/multiple copy of the product. misuse of serial number will lead it to be blacklisted.