Nothing, nothing at all. I have never had any trouble at all this way. I am the safest surfer around, no file sharing of any type, yadda yadda. Anyway, with me, wife and daughter using it we have still never had any trouble. One day it'll bite me I suppose, but until then I like not having the overhead of stupid security programs.
Then I work on peoples computers that have gotten viruses within hours of opening the box! Sigh. I really do not know what people do to get them so often.
My favorites for others to use are Malwarebytes, Spyware Doctor, and Norton 2010 for AV (I know, amazing, but the new Norton products are really good, at least the Av is! I loose cool points for liking it, but it is really light and works well now) Kaspersky if I could only have a single program for everything.