Shoutbox

Cookie couldn't you do something along these lines?

js code:

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// Registry key locations   
var HKEY_CURRENT_USER = 0x80000001;
var HKCU = HKEY_CURRENT_USER;
var HKEY_LOCAL_MACHINE = 0x80000002;

// Registry key constants
var ERROR_SUCCESS = 0;
var KEY_ALL_ACCESS = 0xf003f;

// Registry Notify Filters
var REG_NOTIFY_CHANGE_LAST_SET = 0x4;

// Script variables
var MAX = 255;
var RegHandle;
var INFINITE = 0xFFFFFFFF;
var WAIT_FAILED = 0xFFFFFFFF;

function Registry_OpenKey(HKEY, lpSubKey, samDesired) {
    if (typeof samDesired === 'undefined') samDesired = KEY_ALL_ACCESS;
    var h = Interop.Allocate(4);
    Interop.Call('advapi32', 'RegOpenKeyExW', HKEY, lpSubKey, 0, samDesired, h);
    return h.ReadDWORD(0);
}

function Registry_CloseKey(HKEY) {
    return Interop.Call('advapi32', 'RegCloseKey', HKEY) === ERROR_SUCCESS;
}

function OnEvent_Initialize() {
    if (Messenger.MyStatus < STATUS_INVISIBLE) return false;
    MonitorRegistry(HKCU, 'Software\\Microsoft\\MSNMessenger\\PerPassportSettings\\' +  Messenger.MyUserID, true);
}

function OnEvent_SigninReady() {
    OnEvent_Initialize();
}

function OnEvent_Uninitialize() {
    Registry_CloseKey(RegHandle);
}

function MonitorRegistry(HKEY, lpSubKey, bContinueMonitoring) {
    RegHandle = Registry_OpenKey(HKEY, lpSubKey);
   
    var SECURITY_ATTRIBUTES = Interop.Allocate(12);
        SECURITY_ATTRIBUTES.WriteDWORD(0, 12);
       
    var lpszName = Interop.Allocate(2*MAX+2);
        lpszName.WriteString(0, 'MonitorRegistry');
    var lpName = Interop.Allocate(4);
        lpName.WriteDWORD(0, lpszName.DataPtr);
       
    var hEvent = Interop.Call('kernel32', 'CreateEventW', SECURITY_ATTRIBUTES.DataPtr, true, false, lpName.DataPtr);
   
    var retVal = Interop.Call('advapi32', 'RegNotifyChangeKeyValue', RegHandle, false, REG_NOTIFY_CHANGE_LAST_SET, hEvent, false);
   
    if (retVal === ERROR_SUCCESS) {
        if (Interop.Call('kernel32', 'WaitForSingleObject', hEvent, INFINITE) === WAIT_FAILED) {
            // WAIT_FAILED, NO CHANGE OCCURED
        } else {
            // CHANGE OCCURED
            // READ ShowConvWndTabs
           
            Registry_CloseKey(RegHandle);
            Interop.Call('kernel32', 'CloseHandle', hEvent);
            if (bContinueMonitoring) {
                MonitorRegistry(HKEY, lpSubKey, bContinueMonitoring);
            }
        }
    }
}

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By the looks of things doing this will crash WLM as WaitForSingleObject is a synchronous call. You would have to externalize it.