Based on my experience, I know that the Logon UI affects some components and features on your PC, meaning that if the file is broken or corrupted, certain features won't work as they should, therefore you need to repair your Windows first. It's even better if you have an installation disc with you because the following command will require the install disc, but it can work with the built-in libraries as well, in case you don't have it.
Go to Start and type in the search box: cmd. Right click it and choose Run as Administrator.
Type the following command: sfc /scannow. The command will check all the system files and repair them in case they're corrupted. The PC might restart a few times while you do this and it's recommended to avoid starting any other programs while it's working. It won't happen anything, but just to be sure everything is fixed properly.