Eversince companies like Norton and others started creating AV software, there has been fierce competition in an attempt to dominate the computer protection market. Each product try to outdo the others by providing more and more features to their product. Over time, users like me were finding that even though the speed and capabilities of their computers were doubling every 18 months or so, the necessity to have some form of malware (more general term) protection was effectively cancelling out the computer performance gains from faster processors, memory, and disk drives. In most cases, the effect on older computers was such that the computer was running so slow that it was no longer practical to use the computer! In effect, the AV solution has become the problem and was unintentionally doing a form of DoS (denial of service) against the computer user.
I personally have uninstalled Norton AV and McAfee because they have gotten so bloated that the laptop I had was no longer useable! I have resorted to operational security--meaning that I travel the Internet superhighway effectively naked, only going to places that I know are safe.
With the recent emergence and popularity of netbooks ($200-$300 retail cost), Microsoft (as well as other companies who were already doing the same thing) must have realized that there has to be a huge market for a computer security solution that had the essential features necessary to protect the computer, and yet not bloated to cause noticeable computer performance issues.
Enter Microsoft Security Essentials--a free offering from Microsoft. The product is still in BETA, but Microsoft didn't waste much time in getting it out there. I installed the product about a month ago, and it is appears to be working. However, I've not seen it detect anything. It is either very good, or it just looks like it is running, but not really doing anything. We'll see; I need to find a site that has a means to test the effectiveness of an AV solution.
At least now, I don't feel so naked on the Internet.