I've been banging my head...I can't pretend to be a C++ guy...

    TCHAR * pszUserName = userName.GetBuffer();
SID sid;
SecureZeroMemory(&sid, sizeof(sid));
SID_NAME_USE sidNameUse;
DWORD cbSid = sizeof(sid);

pLog->Log(_T("Getting the SID for user [%s]"), 1, userName);

if (!LookupAccountName(NULL, (LPSTR)pszUserName, &sid, &cbSid, NULL, 0, &sidNameUse))
{
    pLog->Log(_T("Failed to look up user SID. Error code: %d"),1,  GetLastError());
    return _T("");
}

pLog->Log(_T("Converting binary SID to string SID"));

The message 'Getting the SID for user [x] is written' but then the app crashes. I'm assuming is was the LookupAccountName call.

EDIT:

Whoops userName is a MFC CString