You can read more here.

While downloads for the beta have officially been cut off, there are a few other sites on the net you can find it on, just make sure you download the right one for your operating system as there are different versions for Windows XP and Windows Vista/Windows 7, and both also have 32bit and 64 bit variations.

Now, lets take a look at what this looks like once it's installed:

The first thing you notice after installing Microsoft Security Essentials is that you have a new system icon down by your clock:

As you can see, the little blue castle with the white background is the new icon.  Unfortunately, there is currently a minor bug where upon after restarts the icon doesn't show up unless you double click the icon on your desktop.  It can lead the user to believe that the software isn't running and that they system is unprotected.  Hopefully this is something MS can fix before they roll out the software officially.  For those wondering, the software IS running, and you can find it in the task manager.  The executable is msseces.exe.  When you do a scan the system tray icon looks like this: .  A right click option is added to explorer to let you scan files one at a time too

Once you double click on the icon you are presented with the following screen:

This is a very simple page with little user interaction.  If you want to trigger a scan to go off right then, you can select from the 3 options and press the scan now button.  Selecting custom will bring up a window allowing you to choose the drives or folders in which you want scanned. The QUICK option appears to scan the registry, windows, program files and "documents and settings" directories.  It DOES seem to deep scan compressed exe's ( the cabs/zips within them ) and zip files. I have seen it churn away on large compressed files doing it's thing.

From there we go to the Update Tab:

The Update Tab does just what you would expect.  It checks the Microsoft servers for new Spyware and Virus definitions.  Definitions are what the software uses to compare files against their database to flag as either a potential virus or spyware.  Think of it kind of like fingerprint matching.

The History Tab:

Previously I was running the AVG Free Anti-Virus ( AVG ) software. Upon downloading MSE, I uninstalled AVG and installed MSE. I immediately ran MSE and it found 1 virus and got rid of it.  Yes, it was only a javascript vulnerability and I use FireFox to attempt to limit those, but it is still interesting that MSE immediately found something that AVG missed.  I think I have to do a full scan soon.  Depending on the type of infection you can Remove the item ( which in this case MSE did for me ) or Allow the item if you know it is a false positive. There doesn't seem to be a "clean" function that many anti-virus softwares have, but this could be because the fact the software is in beta.

Now to dive into the settings of MSE.  You are given a lot of options, so I'm going to just list them as most of them you'll never want to change except for Scheduled Scans and adding the scan of Removeable Drives.

Overall MSE has all the starts to compete with the big boys and at a fraction of the cost.  What will MSE cost?  NOTHING.  MSE is planned to be a FREE replacement to those paying $50 a year for Microsoft One Care.  The Virus and Spyware definitions are scheduled to be updated as often as 3x a day, so you wouldn't go longer than 8 hours without having access to an update.  The actual software is planned to be updated through Windows Update and assuming that you have Windows Updates scheduled to run every week like you should, then you should pick up all your updates automagically.