Try this:
http://support.microsoft.com/KB/875350
Use Recovery Console
Warning The following steps contain information about modifying the registry. Before you modify the registry, make sure to back it up, and make sure that you understand how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:
256986 (
http://support.microsoft.com/kb/256986/) Description of the Microsoft Windows Registry
If you use Registry Editor incorrectly, you may cause serious problems that may require that you reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
If you cannot successfully remove Windows XP SP2 by using one of the previous methods, follow these steps: 1. Insert the Windows XP startup disk in your floppy disk drive or insert the Windows XP CD in the CD drive or in the DVD drive, and then restart your computer.
Note When you receive the following message, press a key to start your computer from the Windows XP CD:
Press any key to boot from CD
Note Your computer must be configured to start from the CD drive or the DVD drive. For more information about how to configure your computer to start from the CD drive or the DVD drive, see the documentation that came with your computer or contact the computer manufacturer.
2. When you receive the Welcome to Setup message, press R to start the Recovery Console.
Note Multiple options will appear on the screen.
3. Select the Windows XP installation in question.
Note You must select a number before you press ENTER, or the computer will restart. Typically, only the 1: C:\Windows selection is available.
4. If you are prompted to type an administrator password, do so. If you do not know the administrator password, press ENTER. (Typically, the password is blank.)
Note You will not be able to continue if you do not have the administrator password.
5. At the command prompt, type cd $ntservicepackuninstall$\spuninst, and then press ENTER.
Note After you complete this step, you cannot stop the removal process.
6. At the command prompt, type batch spuninst.txt, and then press ENTER.
Note The Spuninstal.txt file appears. As the file scrolls down, you will see errors and files being copied. This is normal behavior.
7. After Windows XP SP2 is removed, type exit, and then press ENTER.
8. Restart your computer in Safe Mode. To do this, press F8 as the computer restarts.
Note After you restart, the system may lock up with a black screen. (Your mouse will work.) In this case, restart again by turning the computer off and then back on. The second restart will let you to log on.
9. When your computer restarts, Windows Explorer (Explorer.exe) does not run, and the Windows icons and the Start button are unavailable. To resolve this problem, follow these steps: a. Press CTRL+ALT+DEL to start Task Manager.
b. Click File, and then click New Task (Run...).
c. In the Open box, type regedit, and then click OK.
d. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\RpcSs
e. On the right side of the viewing pane, right-click ObjectName, click Modify, type LocalSystem in the Value data box, and then click OK.
f. Restart your computer
