If I recall correctly, all of the PC clone manufacturers except for HP reported a decline in their Q3-to-Q4 shipments. Apple did well by maintaining their quarter-to-quarter level compared to Dell, Lenovo, Acer, etc.
I would suggest that a large part of any decline in the late stages of 2006 on the part of PC manufacturers might have something to do with consumers and businesses holding off their purchases to wait a couple of months for Vista. I know many people who did this, and I recommended it to customers who were about to purchase new hardware.