While this question is already quite old and thise answer is probably pointless, this sounds like a monitor calibration issue.
This can usually be resolved in 3 ways. Unfortunately, Brood War doesn't support the 2nd and 3rd way:
- Use the buttons on the actual monitor to move the image on the screen to the left.
- Change the display resolution of the game. Unfortunately this game doesn't support changing the display resolution out of the box: How can I change the screen resolution?
- Run the game in windowed mode. Unfortunately this game doesn't support windowed mode out of the box: Is it possible to run Starcraft in windowed mode?