Hi Blaise170,
If you are using OEM disc to install Windows on your computer, it would not ask for a product key. If you are using a Retail disc, then the product key would be mentioned on the disc or on the package.
If you simply wish to re-install Windows on your computer, you can perform PC Restore. If the system is booting to the desktop screen, you can create System Recovery Discs which you can later use to re-install the operating system on your computer. Re-installing Windows using the Recovery Discs would not require a product key.