WTSSendMessage fail with error Access Denied

i describe my Issue that i am developing virtual printer driver DLL
which is run by
spooler service of operation system, means my driver is run under
spooler service.

Now i need to show one MessageBox from my this DLL in Window vista
but
vista not support any user interface from service thats why i use
this
WTSSendMessage function but this function give me error "ACCESS
DENIED"...


//Code i used


#define WTS_CURRENT_SERVER ((HANDLE)NULL)
#define WTS_CURRENT_SERVER_HANDLE ((HANDLE)NULL)
#define WTS_CURRENT_SERVER_NAME (NULL)


#define WTS_CURRENT_SESSION ((DWORD)-1)


//defination of WTSSendmessage Function pointer
typedef BOOL (*funPtr) ( HANDLE,
DWORD,
LPWSTR,
DWORD, LPWSTR, DWORD, DWORD, DWORD, DWORD*,
BOOL);


funPtr callFun;
HMODULE hModule = LoadLibrary(L"Wtsapi32.dll");
if(hModule)
{
callFun = (funPtr) GetProcAddress(hModule, "WTSSendMessageW");
if(callFun) {
DWORD result;
BOOL ret =
callFun( WTS_CURRENT_SERVER_HANDLE,


WTS_CURRENT_SESSION,


(LPWSTR)L"Thunder Driver",
38,
MB_OK,
FALSE,
&result,
0);


if(!ret) {
Print_Error();
}


}//end getproc
}//end loadlibrary


function return with "Error : Access Denied";;
WTSSendmesage function fail
and return with error "ACCESS DENIED".


This function failed is happening on Window vista only,
One more thing is this same code is running successfully On Window XP
means i think that
there is some problem arise due to Vista securtiy...


So please help me ....
Thanks in Advance....