In Windows XP, some services may attempt to display user interface dialogs or communicate with user applications, which’s a normal funtion. But in Windows 7, your service may experiences one or more of the following symptoms. The service:

• Is running, but cannot do what its supposed to do, and just eats CPU cycles and memory
• Is running, but other processes can't communicate with it and it cannot communicate with the user, or other applications / services
• Is trying to communicate with user applications through window messages, but the window messages are not reaching their destination
• Displays a flashing icon on the taskbar indicating the service wants to interact with the desktop

All the above symptoms point to the conclusion that your service’s experiencing Session 0 Isolation of Windows 7 Services, that’s, the “physical” separation between services and user applications, but more about that in just a bit.

Full Article: Windows blog