Instructions
Log into your Steam account. Click the "Library" tab at the top of the program window to show a list of games tied to your account.
Scroll down to "Borderlands" and right click to open a new menu. Click the "Properties" option at the bottom of this menu.
Click the "Local Files" tab on the new window. Click "Verify integrity of game cache." This allows Steam to scan all the game files located on your hard drive and determine which ones have been changed or hacked. A progress bar will indicate how long this process is going to take. If any changed or hacked files are detected, Steam will automatically re-install an original copy of the file.
Click the "Updates" tab once the game's integrity has been fully restored. Select "Always keep this game up to date" from the drop-down menu under the "Automatic Updates" heading. This allows Steam to automatically update "Borderlands" as soon as a new patch is released.