You could get so many replies to that question!!
In my experience, Microsoft Security Essentials (free) is great if you are, shall we say, "discerning" in your surfing habits. If you don't feel like you can see something shady from a mile away, then like you said, get something stronger. I personally use Nod32 (
www.eset.com) on all my machines, but there are several other decent alternatives. As Arizona Willie mentioned above, Avast is a good choice, and I have heard some mention BitDefender as somewhat decent.
My only gripe about most antivirus, is that they are so flashy, and NAG the user about every. single. thing., that most users have no clue what the AV is trying to tell them, and they very quickly learn to ignore the incessant popup messages coming from the system tray.