If you have an Windows XP with Microsoft Security Essentials Antivirus installed or Microsoft Forefront Endpoint Protection, and your PC is hanging / freezing at the login screen, then the cause is an update that makes it to crash. Also systems with Windows Server 2003 seem to be affected. Definitions version for Microsoft Essentials / Forefront Endpoint Protection is 1.171.1.0.
Uninstall Microsoft Security Essentials or Microsoft Forefront Endpoint Protection in Safe Mode, then you can use your PC and don’t forget to install another antivirus.
Other possible solution : remove the tick for Enable Behavior Monitoring checkbox.