The Windows taskbar is missing after you uninstall SP2 (Service Pack 2) from Windows XP.
This behavior occurs because the uninstall incorrectly modifies the HKEY_LOCAL_MACHINE\System\Currentcontrolset\Services\Rpcss sub-key. When SP2 was installed, the OjectName Value Name string contained NT Authority\NetworkService. Prior to installing SP2, the the OjectName Value Name string contained LocalSystem, but the uninstall failed to set this data value.
NOTE: The exact case is important.
To resolve this issue:
1. Press CTRL+ALT+DEL and press the Task Manager button.
2. In Task Manager, use File / New Task (Run) / CMD / OK.
3. Type REG ADD HKLM\System\CurrentControlSet\Services\RpcSs /V ObjectName /T REG_SZ /F /D "LocalSystem" and press Enter.
4. Shutdown and restart Windows XP.