Steam is allowing free to download and play Borderlands for 3 days in honor of the Borderlands 2 pre-order. Might be a good time to check this awesome game out if you have not already done so.
Just to clarify. You have to have a Steam account and client. When you go to your library you will see a link with a countdown to download and install Borderlands. When the promotion is over your game will be locked.