Each year the leaders in Antivirus software change, depending on how strong they are in protection and how much of your computer resources they consume. MS Security Essentials has come up very significantly. Neil Rubenking at PC Mag rated it very good but not tops. Maximum PC had a great antivirus article in March's issue where it rated MS SE low in features but high in protection. Here is the link:
From my experience with NOD32 and MSE that MSE detected every virus I got along with NOD32. NOD32 really bogged my system down and had firewall issues with certain programs.
I have found other AV/firewall combos to have similar issues, even worse when you consider you pay for them.
I have had no issues with MSE, it runs really well and does not take up all my system resources. It is a very good AV and considering my virus plan, reformat and reinstall, removal is not an issue for me. It is also free.
Microsoft Security Essentials earns its spot as one of the best free antivirus as it has consistently performed admirably on third-party antivirus certifications and detection tests. Microsoft Security Essentials is not only free for home use, it's free for small businesses with up to 10 computers to protect. Microsoft Security Essentials runs on XP, Vista, and Windows 7, but can only be installed on genuine Windows installations (i.e. those that are properly licensed).