Why? Well price is one consideration. It's free. Another good reason is that it works. And finally, it has a small footprint and doesn't utilize a lot of system resources.
|Microsoft Security Essentials|
Now of course being from Microsoft, Security Essentials is designed for the Windows platform. And only works on the newer versions of Windows starting with XP and going up to Windows 7. It does not operate on the new Windows 8 though.
Microsoft Security Essentials replaces the older Microsoft Defender. Until you get to Windows 8 where Microsoft has resurrected Microsoft Defender as an integrated antivirus product. The original Microsoft Defender was engineered to work against adware and spyware. The AV system for Windows 8 is to be a full antimalware suite.
So to sum it up, the reasons I use and recommend the Microsoft Security Essentials is that it integrates with your Windows system, it is free, and it does a fairly good job of keeping your system free of infections.
Now no one single antivirus program works against everything but Microsoft Security Essentials does a fairly good job. And the system integration helps it load sooner than others normally would and it generally is much faster in operation.
Here we are at Tip#2. Tip#1 was about cleaning your PC and Tip#3 will be about removing and immunizing against spyware. Stay tuned.
Microsoft Security Essentials
Wikipedia on Microsoft Security Essentials