From the site:
Minimum system requirements for
Microsoft Security Essentials
Operating System: Genuine Windows XP
(Service Pack 2 or Service Pack 3);
Windows Vista (Gold, Service Pack 1,
or Service Pack 2); Windows 7
For Windows XP, a PC with a CPU clock
speed of 500 MHz or higher, and 1 GB
RAM or higher. For Windows Vista and
Windows 7, a PC with a CPU clock speed
of 1.0 GHz or higher, and 1 GB RAM or
higher. VGA display of 800 × 600 or
higher. 140 MB of available hard disk
space. An Internet connection is
required for installation and to
download the latest virus and spyware
definitions for Microsoft Security
Essentials. Internet Browser: Windows
Internet Explorer 6.0 or later.
Mozilla Firefox 2.0 or later.
Microsoft Security Essentials also
supports Windows XP Mode in Windows 7.
For more information see the system
requirements for Windows XP Mode in
Windows 7
