According to powerleap Intel manufactured some rare P4 2.8GHz 400MHz FSB 478pin 512K Northwood CPU with hyperthreading (RK80532PC072512).
I recently purchased one and it runs well on my machine with intel d845hv mobo (faster than my old p4 1.6). But I cannot seem to find a way in the bios to enable hyperthreading. (Nor do I see the virtual CPU displayed in my task manager or device manager). Does anyone know what I can do? The latest bios supposedly has Machine Check work around for HyperThreading...whatever that means.