Microsoft has released a beta version of their free security software today. The limited beta is limited to 75,000 downloads. So this is very very limited on the number they are giving out. I have grabbed a copy for my netbook.

Quoting LifeHacker

Also according to ZDNet, you’ll need to be in the U.S., Israel, or Brazil to try out the beta, and like most early downloads from Microsoft, you’ll need to sign in with a Live account to get rolling (I also had to complete a Microsoft Connect registration and a survey). MSE is available for Windows XP, Vista, and Windows 7 (including both 32- and 64-bit for Vista and Windows 7).

Another thing worth noting is you must have a genuin version of Windows.