[[__hector]]

No lo he probado en .NET, pero puedes intentar llamar el API getsysteminfo de la dll kernel32.dll (api del sistema operativo)

En c# va mas o menos asi :

using System.Runtime.InteropServices; 

//Estructura que sera parametro de la funcion 
[StructLayout(LayoutKind.Sequential)] 
public struct SYSTEM_INFO 
{
 public uint dwOemId;
 public uint dwPageSize;
 public uint lpMinimumApplicationAddress;
 public uint lpMaximumApplicationAddress;
 public uint dwActiveProcessorMask;
 public uint dwNumberOfProcessors;
 public uint dwProcessorType;
 public uint dwAllocationGranularity;
 public uint dwProcessorLevel;
 public uint dwProcessorRevision; 
}
 
[DllImport("kernel32.dll")]
static extern void GetSystemInfo(ref SYSTEM_INFO pSI); 

// y de aqui lo invocamos

protected void button1_Click (object sender, System.EventArgs e)
{
 SYSTEM_INFO pSI = new SYSTEM_INFO();
 GetSystemInfo(ref pSI);
 
  Response.Write(pSI.dwActiveProcessorMask.ToString());
}