I assume big computer stores (ie. Best Buy, Fryes) don't have permission to modify any PC components before they're sold. They can package them with other external products but they can't change video cards, etc. before the PC's sold. I would then assume by the time the big stores get their shipments of new name-brand PCs in, they're technically all OEM at that point. People who build their own or modify their bought PCs wouldn't be covered by any OEM warranty or policy.
I guess OEM has more to do with a PCs' warranty coverage than whether it's full of bloatware or not.
Thanks for clarifying that for me Nergal.