Currently patching 13,000 POS systems in the field due to this. Thanks, Microsoft.
http://hothardware.com/m/News/Coincidence-Or-Accident-Microsoft-Security-Essentials-Bug-Rendering-XP-Machines-Hamstrung-Like-A-Virus/default.aspx
http://hothardware.com/m/News/Coincidence-Or-Accident-Microsoft-Security-Essentials-Bug-Rendering-XP-Machines-Hamstrung-Like-A-Virus/default.aspx