I have a very strange and very annoying issue!!!

When certain users try to lock the computer using either Ctrl-Alt-Del, WinKey+L, or the Start Menu shortcut nothing happens.

Other times these users are able to lock the computer only for it to automatically unlock without them entering a password or for them to just press Ctrl-Alt-Del and again unlock it without a password.

There are also occasions when they will press Ctrl-Alt-Del and the Ease of Access Centre is displayed for no apparent reason.

There doesn't seem to be any pattern to this behaviour - all the machines have the same Windows 7 Enterprise image and all the users have the same GPO restrictions. Some users seem to be completely unaffected by this.

Any ideas would be much appreciated...