Something like the ECS DeskNote would be sort-of what you may want. As the name implies, this isn't the most portable notebook. By default they come without a battery. Some merchants will even sell them without certain things like memory and hard drives, and they almost always come without an OS loaded.

As far as CPU goes, I am quite certain the P4 versions use a socketed processor, so you could probably buy a low end CPU model and upgrade it.