Only 1 CPU appearing in task manager on Windows 2003 Server even though 2 are added in VCenter.
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If you've tried rebooting the server and the second CPU is still not showing up in task manager, follow these steps:

1. RDP to server
   
2. Open Device Manager
   
3. Expand > Computer
   
4. In the sub list you will see something other than ACPI Multiprocessor PC e.g:
   Standard PC
   ACPI Uniprocessor PC
   etc
   
5. Right click the item under Computer
   
6. Select > Properties > Driver > Update Driver
   
7. Choose:
   No, not this time > Next
   Install from a list of specific location > Next
   Don't search. I will choose the driver to install > Next
   
8. Untick "Show compatible hardware"
   
9. Select:
   ACPI Multiprocessor PC
10. Click: Next > Finish
11. Reboot the machine. The second CPU will now work
    

If ACPI Multiprocessor PC is not available in the list, follow these steps:

1. In the RDP session, download the following file to the machine with the issue:
   http://kb.hyve.com/attach/ACPI%20multiprocessor%20PC%20driver/devcon%20-%20app%20for%20multiCPU%20issue%20win2k3%20%26%20vmware.exe
2. Extract: devcon.exe
   
3. Copy to: C:\Windows\system32
   
4. Run the following commands:
   

SET HAL=ACPIAPIC_MP
devcon.exe sethwid @ROOT\PCI_HAL\0000 := !E_ISA_UP !ACPIPIC_UP !ACPIAPIC_UP !ACPIAPIC_MP !MPS_UP !MPS_MP !SGI_MPS_MP !SYSPRO_MP !SGI_MPS_MP
devcon.exe sethwid @ROOT\ACPI_HAL\0000 := !E_ISA_UP !ACPIPIC_UP !ACPIAPIC_UP !ACPIAPIC_MP !MPS_UP !MPS_MP !SGI_MPS_MP !SYSPRO_MP !SGI_MPS_MP
devcon.exe sethwid @ROOT\PCI_HAL\0000 := +%HAL%
devcon.exe sethwid @ROOT\ACPI_HAL\0000 := +%HAL%
devcon.exe update %windir%\inf\hal.inf %HAL%
devcon.exe ReScan

Ensure that the "devcon.exe update %windir%\inf\hal.inf %HAL%" command does not report a failure. 
If it does fail, it may be necessary to temporarily install another HAL either from Device Manager 
or by changing the SET HAL=ACPIAPIC_MP line and re-running the script. 

You will now be able to select: ACPI Multiprocessor PC as discussed above.
Reboot the machine and the second CPU will now work.