With the amount of false flagging I've seen lately from various virus checkers, I was wondering when a time would come that it one would flag a critical system file. For example, every time my work colleague carries out an exam, she has to disable AVG as it keeps claiming the testing software has a trojan (while other AVs find no issue). Nod32 occasionally flags the SIW utility as an infection and the same with the setup file for ExactAudioCopy as adware.
I've used Microsoft Security Essentials on my PCs and recommended it to friends and so far had no issue with it. It definitely does not hog the system as much as McAfee or Norton on low spec PCs such as atom-based and netbooks. As a free virus checker, it's also the only one I know of that is not nagware, i.e. bringing up notices like "(AV) is about to expire! Limited offer: Upgrade to our professional version for only $xx now." with each new version and hiding the free download links.
I currently use Nod32 on my main PC, but I'm probably going to switch to Microsoft's one once it expires. When I bought Nod32, it was light weight virus checker that claimed to have no noticeable impact on system performance. At that time, I was definitely happy with it on my old AMD socket-A based PC. Ever since v4, it's quite a CPU hog, taking a long time to scan certain files. For example, it takes 10 seconds to copy the SIW.exe utility from one drive to another on my i5 based PC, due to it taking that long to analyse it in the background. It also interrupts large downloads in Firefox (>50MB) and the only workaround was for me to disable its browser integration.