There are three levels of preset power management in Vista ... Balanced, Power Saver and High Performance ... You can find further reading in this Microsoft Article. I personally recommend that you actually customise the settings other than relying on any of the presets. None of them is exactly perfect for most systems.
To customise these settings to your own needs, follow these steps.
Open power options by simply typing Power Options in the search bar from the start button. Or, go the long way and, click the Start button / Control Panel / Hardware & Sound / Power Options. ;-)
Click the "Create a power plan" in the left pane.
Press Next to continue ...
I do NOT recommend enabling the "Put the computer to sleep" function.
Now press the Create button ...
As you can see, we now have our new power plan ready to further tweak. Now simply click "Change plan settings" to get to the more detailed settings
The setting you want is display....
Let me know if this works for you !!!!
What is happening is the power settings are shutting off the monitor at a time you set.
You may also want to check your screen saver settings, that would do it as well....
Just right click on your screen, (not a window), and choose personalize, and choose screen saver, and see what that is set up as.
See if that works for you as I'm sure it's one of those settings.
You might try setting to power settings to not turn off the monitor at all.
You may also need specific monitor drivers as well if the power settings have no effect.