Some of these things are a little impractical; the degree will depend upon your particular situation. That said, the more of these things you can do, the better off you (and your computer) will be.
Schneier on Security: Safe Personal Computing
I am regularly asked what average Internet users can do to ensure their security. My first answer is usually, “Nothing–you’re screwed.”
But that’s not true, and the reality is more complicated. You’re screwed if you do nothing to protect yourself, but there are many things you can do to increase your security on the Internet.
Two years ago, I published a list of PC security recommendations. The idea was to give home users concrete actions they could take to improve security. This is an update of that list: a dozen things you can do to improve your security.