Recent definition updates for Microsoft Security Essentials caused some Windows XP machines to not boot. This issue has now been fixed by Microsoft.
Microsoft Security Essentials made Windows XP machines un-bootable
Microsoft has released its Antimalware Engine 1.1.10502.0 to all Microsoft Security Essentials, Forefront Client Security, Forefront Endpoint Protection, Windows Intune Endpoint Protection and System Center Endpoint Protection software on 17 April 2014 along with Signature package 188.8.131.52. This update caused some computers running Windows XP and Windows Server 2003 to not boot.
The workaround was to boot into Safe Mode and uninstalling MSE or simply disabling Behavior Monitoring.
But Microsoft has now fixed this issue:
This was corrected via signature update, which automatically resolved the issue, and customers who have deployed the most recent signatures do not need to take any action. Customers who deployed workarounds such as disabling Behavior Monitoring or any other features should revert those changes.
While Microsoft has stopped supporting Windows XP, definition updates for Microsoft Security Essentials will continue till July 2015. Since 26th of March, 2014, Microsoft Security Essentials had started warning Windows XP users about support ending for this old operating system.
While it is always recommended to a newer operating system, if you feel that your upgrade decision may get delayed for some reasons, you might want to have a look at our posts on securing Windows XP after End Of Support and Windows XP End of Life recommendations.