Is it the best antivirus software out there? I don't know, but the major AV software testing organizations rate it pretty highly. (I actually looked at other sites, as well, but this one (Virus Bulletin @ virusbtn.com), has a very handy comparison chart)
Is it the most-compatible with all (or very nearly all) PC's? Unsure, but it would make sense that it should be, as it is from Microsoft.
Is it really nice to see that it compares well with the other, more well-known software, but is FREE? I sure think so!
I have been using McAfee Antivirus for many years... simply because that was what was pre-loaded onto the first PC that I bought way back when! I downloaded and tried using Norton Symantic one time, because I found it cheaper, but that was a huge mistake, as it was even more HUGE and bulky than McAfee, and caused even more system lock-ups. I removed it and went back with McAfee within only a few days.
Over the past couple of years, I have noted that McAfee was conflicting with several of my other software drivers, even though I keep my system fully-updated with each new driver or software release. It was also becoming ever-larger, as they added additional features to the package (nearly 59 megs last version), and demanding more and more of my system resources.
This past version (2009) was the very worst ever! Because of these aggravations, I decided that when my annual subscription ran out, this time, that I was going to go with a different AV. I had looked any number of times in previous years, and some of the names on my mind from those earlier researches were popping back up this go-round. These included: AVG, avast!, Avira AntiVir, AVK (G DATA), Eset NOD32, Kaspersky, Panda, etc. However, buried right in there with them was Microsoft's Security Essentials, and it's numbers were reasonably close to these highest-end AV's.
BUT, it was notably smaller (only about 8.6 megs), and it was FREE!
Upon researching owner comments, ratings, and reviews online, I found that it was good enough for me. It is actually a better-rated product than the McAfee I'd been using for so long, and proved it shortly after install by intercepting 3 trojans! (Honestly, I cannot recall McAfee's junk ever giving me a single alert)
I give it 4 stars only because I have just had it installed for a bit more than a week. Otherwise, I feel certain this would be an easy 5-star-rating. It was a very fast and easy download. (It actually took notably longer to uninstall the McAfee junk than to install, set-up, and run the new software!) It runs in the background, and even while doing an active scan, it does not 'drain' my system in the way McAfee did, PLUS it completes the same scan in waaaaay less time (honestly, it was at least 30 minutes for McAfee to complete (when it didn't cause an application hang--- and give me an error message that it had encountered a problem and could not complete--- McAfee took between 30-45 minutes to scan approximately 45000 files; Security Essentials does it in 20 minutes, consistently!) All my application hangs, software/driver incompatibilities, and the ensuing aggravation and time consumption: GONE !
I like it !