You try to start a computer running Windows 7 or Windows Server 2008 R2. However, the operating system stops responding at the "Please wait" screen that appears before you'e requested to press Ctrl+ALT+DEL. Therefore, you cannot log on to the computer. "This issue occurs because an auto-start service remains in the pending mode for too long and cann't start during the Windows startup process. This's caused by a deadlock that occurs because of an interaction between Windows Error Reporting (WER) and AppLocker," explains Microsoft in KB983551.
A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article.