MSE works flawlessly on my antiquated machine with XP and 1.5G RAM. I'd never heard of any concern over XP until reading the replies to this thread.
Microsoft Security Essentials uses smart caching and active memory swapping so signatures that are not in use are not taking up space, thus limiting the amount of memory used even as the volume of known malware continues to increase. This makes Microsoft Security Essentials friendlier toward older PCs, as well as today?s smaller, less powerful form factors such as netbooks.
Minimum system requirements for Microsoft Security Essentials
Operating System: Genuine Windows XP Service Pack 3 (SP3); 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.