We explained one reason for this behavior in tip 0667.

You must have checked Allow Service to Interact with Desktop on the Spooler service Startup.

1. Control Panel / Services.

2. Scroll to the Spooler service and select it.

3. Press Startup.

4. Select the System Account in the Log On As group.

5. Select the Allow Service to Interact with Desktop check box.

6. Stop the Spooler service.

7. Start the Spooler service.