I'd suggest you just stick to walking up and down the stairs during your break, and then you do some real lifting after work when you said you might be able to hit the treadmill. Maybe hit the treadmill once or twice a week, and make time for hitting the weights at least twice a week. In my opinion, if you have to choose between the treadmill and lifting, and assuming you have your diet squared away, lifting is going to provide you with an overall better body composition much quicker.
Mark has a bodyweight program, you could just follow that to start with. I do tons of bodyweight work, and it's a great way to get started and stay fit. After learning your way around the gym, you might want to consider transitioning to weights focusing on the major lifts (squat, deadlifts, bench, overhead press, pull ups [or rows if you can't do pull ups]).