Remember it is anti-virus only, so you would need to activate Windows Firewall inbound as well as outbound and it can be a total mindbender getting it configured. Plus you should keep onboard some anti-spyware applications just in case.
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 256 MB 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.
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