Help with threads

Hi All,

I'm trying to write a windows service by modifying an example I found
at http://www.thecodeproject.com

This is my first encounter with a multithreaded app so please forgive
this newbie question but I'm having a little bit of difficulty using
global variables inside the threads

The code basically looks like this

HANDLE killServiceEvent;
HANDLE hGinaThread, hTrayIconThread;
char * szPinNumber;

void main() {
DWORD idGina, idTrayIcon;

hGinaThread=CreateThread(0,0,(LPTHREAD_START_ROUTI NE)GinaThread,0,0,&idGina);

hTrayIconThread=CreateThread(0,0,(LPTHREAD_START_R OUTINE)TrayIconThread,0,0,&idTrayIcon);
WaitForSingleObject(killServiceEvent,INFINITE);
CloseHandle(killServiceEvent);
}

DWORD GinaThread(LPDWORD param) {
//Code gets pin number from a mailslot
szPinNumber = szMailslotData;
return 0;
}

DWORD TrayIconThread(LPDWORD param) {
Sleep(120000);
MessageBox(NULL, szPinNumber, NULL, MB_OK);
return 0;
}

The program just crashes when it tries to display the MessageBox.

However if I change the code slightly by changing the declaration of
szPinNumber to char * szPinNumber = "PIN Number"; and comment out the
line in GinaThread that updates szPinNumber the code executes
flawlessly.

Any ideas?