How do I run Borderlands in windowed mode?
I was wondering the same thing. Turns out, it's really easy. You just need to add -windowed to your launch options.
In Steam:
Right-click the Borderlands entry in your library.
Click Properties.
Click Set Launch Options...
Enter
-windowedin the box.
Alternatively, you can use Alt-Enter while in the game to switch between full-screen and windowed mode.