After you remove Windows XP Service Pack 2 from a Windows XP-based computer, the Windows taskbar is missing.
This issue occurs when the following registry subkey is not modified correctly:
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows
To resolve this problem, follow these steps:
Press CTRL+ALT+DEL, and then click Task Manager.
In Task Manager, click File, click New Task (Run).
In the Open box, type regedit, and then click OK.
In Registry Editor, locate and then click the following registry subkey:
In the right pane, right-click ObjectName, and then click Modify.
In the Value data box, type LocalSystem, and then click OK.
Note You must type "LocalSystem" exactly as it appears in this step. If you do not type this term correctly, Windows Explorer (Explorer.exe) does not run, the Windows icons are unavailable after the computer restarts, and the Start button is unavailable after the computer restarts.
Restart your computer
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
875350 How to remove Windows XP Service Pack 2 from your computer