I've been using Microsoft Security Essentials for more than a year, since the earliest beta builds. All of the PCs in my house run it, and it protects my home business.
I recommend it to my clients and friends. I even rewrote several big chunks in "Windows 7 All-in-One for Dummies" to vouch for Microsoft Security Essentials over any other antivirus product. It's that good.
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The big problem with Microsoft Security Essentials? Although it's free for personal use, Microsoft has had no method for companies to get with the system. (One exception: The Microsoft Security Essentials license allows installation "on your devices in your household for use by people who reside there or for use in your home-based small business.")
Until now -- effective this week, Microsoft officially allows small businesses to download and install up to 10 copies of Microsoft Security Essentials for free. The only requirement is that you have legal "genuine" copies of Windows, including Windows XP -- which is more than you can say for Internet Explorer 9. Microsoft Security Essentials works great on XP, too.
Most people don't realize that Microsoft Security Essentials has a user interface; they just install it, then forget about it until a message pops up saying it's saved their bacon. If you're running Microsoft Security Essentials, you might find it instructive to open the program from the Start menu or the icon down in the notification area near the system clock. Choose the Settings tab. There you'll find options to set a restore point prior to scans or enable scans of removable drives during full scans. You can also exclude specific files or file types. You can review your PC's scan history as well.