The STB does not have a VGA output. Depending on the model (HD or not) you will have composite, S-video, component, and HDMI (as well as possibly RF - again depending on model).
If your VGA monitor won't accept any of these signals you are out of luck.
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Keyboards is right about the inputs. One option you do have is to purchase a TV tuner for your PC. You can hook you STB into the tuner card and from there via the normal VGA to the monitor. I have one from Hauppauge, and there are others from ATI. The card I have is also HD capable and the picture is great.