I've used Microsoft Security Essentials for well over a year now. I started off testing it to destruction but couldn't find anything that got past it, then recommended it and installed it in hundreds of customers' systems since then.
It is M$ software that's hacked first so it follows that they trace the threat first and it's likely they will update their product before tipping the other AV developers off.
It's free - you paid for it when you bought Windows - and comes in 32 and 64 bit versions at Microsoft's website.
although there are quite a few dependable av/is solutions on the market today i trust nothing but kaspersky internet security on my own personal pcs.
-low memory usage
-not buying a renewal does not shut the program off (norton does! which is bad)
-new version of kis does not generate as many popups and is easier to use
-reliable in terms of how well it stops threats.
look at any AV rating chart, its in the top few always.