If you purchased an Nvidia Card during their free Borderlands 2 Promotion, you can enter the code you have received at the following website:
http://www.geforce.com/games-applications/pc-games/borderlands-2/de...
Please note this site is owned and maintained by Nvidia as is the promotion. If you have issues using the site or with redeeming the code you will need to contact Nvidia Support directly for further assistance.