I always adjust the HAL to correct this issue.  With Windows 2003 you used to need a special patch, but it is now part of SP2.  If you're not running SP2 then you can call M$ and ask for KB923425 which allows you to choose Choose ACPI Uniprocessor PC .  I wrote a whole document on this for our company.  I will paste it in for you, but of course it won't have the screenshots.

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Changing the HAL from Multiprocessor to Single processor after P2V conversion:

1.     Before making any changes to the VM, Create a Snapshot in Virtual Center.

2.     Right click on the VM you want the Snapshot for and go to Snapshot

3.

4.     Click Take Snapshot

5.     

6.     Name it and click OK

7.     

8.     Snapshot will create

9.     

10.     RDP or Console into the Server and go to Device Manager

11.     For Windows Server 2003, install the following hotfix.  It does not require a reboot.  This hotfix allows you to downgrade to Uniprocessor.  WindowsServer2003-KB923425-v2-x86-ENU.exe

12.     Expand the Computer tab to see what HAL is loaded

13.     

14.     In this case, this server used to have 2 processors when it was in its physical form.  This needs to be changed.  To do so, Right click on ACPI Multiprocessor PC and choose Properties

15.     

16.     Click the Driver tab

17.     

18.     Click Update Driver

19.     

20.     Click Next

21.     

22.     Choose Display driver so I can choose and click Next

23.     It will show you this screen

24.     

25.     Choose “Show all hardware of this device class”

26.     

27.     Choose ACPI Uniprocessor PC and click Next

28.     You will be warned about changing this driver, but don’t worry, if it fails, we can revert from the Snapshot we took!

29.     

30.     Click Yes to continue

31.     

32.     Click Next

33.     

34.     Click Finish

35.     Click Close

36.     You will be asked to reboot the computer.  Here is where we will find out if it boots cleanly!

37.     

38.     If the Computer reboots cleanly and everything seems to work, clean up the Snapshot you took.  Verify in Device Manager that the HAL has changed.  This should really improve the VM’s performance!

39.     To cleanup the SnapShot, Open Snapshot Manager

40.