It doesn't matter if the PC is custom built or not. Actually all PCs are custom built. Even Dell, Compaq and IBM puts random stuff in their machines and sell them as exactly the same model. At least IBM has a seperate type signature which tells that it's slightly different internally. I've opened brand PCs and sometimes they had seagate HDs, sometimes Maxtor, sometimes IBM, and although they all had 40GB, the quality and life-expectation of those HDs is completely different. Some had GeForce, ATI, Video7 cards, but the specs said only they had a SVGA-capable video card with 8MB memory, and still the model of PC was the same.
And there can be a tool which tells you which Class your PC is when you run it.