Using Windows these days just seems like asking for trouble on the spyware/malware front. Some of it is your machine getting owned, other times its just missing a checkbox on a program install that installs a browser bar that does God knows what.
My Macbook Air is pretty much my computer of choice these days (with very little third party software). Soon Apple will require sandboxing on the Mac App Store too. Linux of course also being a much better choice than Windows from a security point of view.
On the browser front I can't see myself using anything other than Chrome. Part of this is feature-related and the whole one process per tab thing but it's also the most security-conscious browser IMHO (disclaimer: I work for Google but not on Chrome).