Fact 1. ADC is just DVI plus power and USB.
Fact 2. DVI->ADC adapters, such as Apple's, add on the power needed by the display, for about $100.
Fact 3. VGA->DVI adapters (the kind that actually convert analog to digital, as required by a DVI LCD) exist, but cost as much as that display is worth.
Fact 4. Devices that combine both adapters do exist -- the best (only?) example is Gefen's
VGA to ADC Conversion Box for $299.
Of course, for that much money, you could just buy a 17" VGA LCD for the PC.
The adapter included with a PowerBook does not do what you need -- it takes DVI-I
output (the I means that it contains both digital and analog signals) and strips out the analog to a VGA connector.
tooki