fix computer errors

My WinXP Media Center PC recently started giving me an error message on startup. The message says:
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.
I haven’t installed any new software recently, and I’m careful when surfing the web (Firefox+ZoneAlarm Internet Security Suite). Any idea what is going on here?