If it is a branded PC, go to the manufacturers web site find the Support/Drivers option and stick in your make and model number.
The next easiest option is to take the lid of the system uint and look at the motherboard, you should be able to see the name written on the graphics chip or the identification of motherboard itself. Then just Google that.
Some PC's display the mother board Id number in the bottom right hand corner of the screen for a few seconds when booting up just as they go through POST. I have used a digital camera on a number of occasions to capture the number. Again you can just type this into Google and get the motherboard details including those of the embedded graphics etc.