It probably is not going into 'sleep'. just a blank screen means no video.Make sure you remove ALL the old nvidia drivers before installing new ones.You can do that in windows explorer, Nvidia > drivers.Then the machine will run on AGP drivers until you install the next driver.
If its an AGP slot card you need to use one of the older drivers as the new ones are not too hot, even with windows 7.
Device manager > display, right click > properties to get to the drivers tab.

In windows power settings, you ajust the time the monitor runs and shuts down one setting is do you want the monitor to stop after 5,10 or so mins you answer never that will cure the shut down