While this question is already quite old and this 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:

1. Use the buttons on the actual monitor to move the image on the screen to the left.
2. Change the display resolution of the game. Unfortunately this game doesn't support changing the display resolution out of the box: 
https://gaming.stackexchange.com/questions/103933/how-can-i-change-the-screen-resolution
3. Run the game in windowed mode. Unfortunately this game doesn't support windowed mode out of the box: https://gaming.stackexchange.com/questions/114838/is-it-possible-to-run-starcraft-in-windowed-mode