So find out why I did the same: http://www.askwoody.com/2009/microsoft-security-essentials-da-facts/
For more about Microsoft Security Essentials see this MS press release.
To download the program go to http://www.microsoft.com/security_essentials/.
Be sure before you install MS SE to remove any other antivirus software you have installed: whether it's Norton's (Symantec), McAfee, AVG, Avast or any other. Don't run more than one antivirus program at the same time or your whole system can crash.
OK, I still double (triple and quadruple) check with Spybot Search & Destroy but I disabled the resident TeaTimer because you don't want too many resident programs running in the background, bumping into each other. I also use Ad-Aware without the resident Ad-Watch and Malwarebytes Anti-Malware (free, non-resident version).
IMPORTANT: One caveat. One PC expert told me
Remember that installing and uninstalling software is NOT good for Windows. especially stuff that works intimately with the OS itself like antivirus stuff. it's just asking for problems playing around with it.So if you already have a good antivirus program installed, you might just want to leave well enough alone. On the other hand, if you don't have a good AV program installed or you're just paranoid, this looks like a good option.
As for me, I have a lot of alligators in my moat. You can't be too careful these days. It's my personal Homeland Security system.