Nothing. And I mean *nothing*. No antivirus, no other placebo "I will make your PC 10% faster and more secure" crap (in reality, every additional software running will make the computer slower).
It's all a matter of knowledge and how one is using a computer. After a clean boot, my Win 8 uses around 500MB RAM, with 23 (or so) processes running. Clean, fast, simple. I don't like to spend hundreds on hardware only to waste all the resources with useless background crap.
There are basically only two things I do every now and then:
a) defragmenting the system partition
b) cleaning up, compacting and defragmenting the registry.