I ran across a link (in German) on the improvements made in the One Care/Forefront anti-virus engine and its signature database its apparently now as complete as the "competition" (translation by Google).
This is great, I do not (or at least I think) drink the Microsoft cool-aid but so far in my experience is that One Care is far less intrusive and negative to systems that the competing products; its very sad that I have gotten to removing AV from machines as one of the diagnostic steps I take if the machine behaves badly; to-date I have not had this problem with One Care.
That being said One Care, like most Anti-Virus scanners consumes a lot of memory, so in some cases I have no choice but to leave AV off.
While on this topic I do have to say the LavaSoft AdAware product is AWSOME from a anti-malware scanner, I usually put Windows Defender on machines but if I find a machine that did not have Windows Defender on it the AdAware stuff is always a quick and easy way to clean a machine up; I should make a personal note to look at how defender does with this initial case as I usually run AdAware 1st then install Defender.