XP Logon Screen fix after 2014 Updates! Root Cause is Microsoft Security Essentials!!!
No worries, there is an easy fix!
Reboot the machine into Safe Mode (F8 key while the machine is booting up)
Go into Control Panel, then Add/Remove Programs, select show updates, and remove all of the April 2014 updates
While in Control Panel, navigate to System, then Updates and TURN OFF Automatic Updates
Navigate to C:\Program Files\Microsoft Security Client folder. Delete all of the files in this folder, there will be one or two files that you can not delete.
Rename these files to .txt
Reboot your XP machine, and you should be able to login as normal!