Go into the folder titled 'dlc', and find a document titled, 'willow.' Before editing, make a backup. To get Golden Keys when you don't already have any, you have to revert Borderlands 2 for PC to the unpatched, vanilla version, and go into the install directory. When you start again, all your golden keys will be back! Once you're done getting all the free gear you want, quit Borderlands 2 and set the profile.bin file back to normal. Then load up Borderlands, spend your golden keys, and quit the game. Make a backup copy, then set the original to read-only (right-click, properties, Read-only checkbox, Apply).
In that folder you'll find a file named profile.bin. To get unlimited free golden keys, you have to have 1 or more already in stock, and go to your Borderlands 2 save games folder (usually My Documents/My Games/Borderlands 2/WillowGame/SaveData/#).
