I've got exactly the same problem with a brand new laptop. Running Windows 7 Pro x64, OOBE setup so I still have all the Acer crap on it.
Problem first came about when the user would 'wake' the machine up, press ctrl-alt-del, the screen would wipe the login message to display the password fields but never displays them.
I have the blue windows background, with Windows 7 Professional logo on the bottom center and the ease of access button on the bottom left hand side. This button does nothing, mouse and num-lock are still active. Cannot shutdown or cancel out of it, only option is to do a hard shut down with the power button.
This issue happens on start up, restart or coming out of hibernation or after locking the workstation. Fully updated with Windows and Office updates. Disabled hibernation and removed acer power management to no afffect.
Very very annoying. Sometimes restarts fix the issue but other times it doesn't. Laptop is on a domain.