WMI enables you find the number of processors in your system:

PS> Get-WmiObject -Class Win32_ComputerSystem | fl Number*

NumberOfLogicalProcessors : 2
NumberOfProcessors        : 1

This works fine for Windows Vista/Windows 2008 and above.

Earlier versions of Windows mis-report the number of processors – it counts the number of logical processors reports it as the number of physical processors.

Win32_Processor has the same problem on Windows 2003 and below.

There is a hotfix available from http://support.microsoft.com/kb/932370 that will correct the behaviour of these two WMI classes so that they report correctly