[hasherfrog]

Цитита:

It is also possible to disable the pop-up dialog boxes. To do this, you must stop the Messenger service as follows:

  1. In Control Panel, double-click Services.
  2. Select the Messenger service.
  3. Click Stop.

-or-

Run the following at the command prompt:

net stop messenger


Ты случайно этого не делал?
В общем, проверь, не остановлена ли служба Messenger.