Microsoft Security Essentials is getting better with every update, it is light, fast and the detection rate is reasonably good. Give them another year or two, and a few more releases and I bet Microsoft Security Essentials will probably be one of the best antiviruses in the world.
Yesterday I was having problem updating my Microsoft Security Essentials definition, probably a bug. If you happened to be in the same situation or need to update your antivirus definition because you don’t have internet access for certain computers, do download and update the definition manually. Install the latest Microsoft Security Essentials definition updates. It is important to keep your antivirus definition up to date.
p.s. Microsoft Security Essentials lacks basic features such as deleting malware that Microsoft SE has detected or hasquarantined and many more. This antivirus is for beginners, not amateur.