I have a Windows Server 2008 machine running in VirtualBox. On initial installation, only one processor was made available, but now I want to run it as a multiprocessor machine.

I have made all four cores available in the VirtualBox settings (as well as enabling VT-x/AMD-V and Nested Paging), but Task Manager still only shows one CPU. However, the four CPU cores are visible in Device Manager under Processors.

In the event log on startup, I can see the following relevant events:

EventLog.6009 Microsoft (R) Windows (R) 6.00.6002 Service Pack 2 Multiprocessor Free
Kernel-Processor-Power.4 Processor 0 exposes the following: 1 idle state(s), 0 performance state(s), 0 throttle state(s)
Kernel-Processor-Power.4 Processor 255 exposes the following: 0 idle state(s), 0 performance state(s), 0 throttle state(s)
Kernel-Processor-Power.4 Processor 255 exposes the following: 0 idle state(s), 0 performance state(s), 0 throttle state(s)
Kernel-Processor-Power.4 Processor 255 exposes the following: 0 idle state(s), 0 performance state(s), 0 throttle state(s)

How can I make this system actually boot up as a multiprocessor machine?