Is it a prebuilt system??? Gateway, Compaq etc. Or a home or shop built system???
Prebuilt.... Brand and model number, preferably from a label on unit and not just what it says on the front.
Home or shop built.... look at the motherboard and see what name or model info is printed directly on the board or on a label.
Here's an example on a Gigabyte board, sorry for the crappy picture but it's what I had to start with.http://img10.imageshack.us/img10/5390/motherboarsproductimage.th.jpg
Click picture for a larger image.