If, when your Windows XP SP2 boots up, you get a popup with the following error:

RTHDCPL.EXE – Illegal System DLL Relocation

The system DLL user32.dll was relocated in memory. The application will not run properly. The relocation occurred because the DLL C:\Windows\System32\Hhctrl.ocx occupied an address range reserved for Windows system DLLs. The vendor supplying the DLL should be contacted for a new DLL.

Then probably, you have Realtek HD Audio Control Panel installed and had your windows updated recently. The issue was already assumed by Microsoft. You can get additional information and download a patch here:  http://support.microsoft.com/kb/935448/.