Using Windows7 with one 'Admin' account which I rarely use, and one regular account with UAC set to highest security level.
This means every time you install something, or move files in system directories, UAC will lock up your system and ask for your Admin password.
A lot of people find that annoying and disable it, but that's really the single biggest security risk you can take.
If you use other OS such as OSX or Linux this is the usual way of doing things.
I also run startup monitor.
http://www.mlin.net/StartupMonitor.shtmlI browse using Chrome and Adblock, and I use MS security essentials which meshes nicely and is unobtrusive.
I do not use MSN, it attracts all kinds of crap, you can use an alternative such as Pidgin if you want to avoid using it.
I also remove useless software from time to time, rather than letting it bloat up.
I'm pretty happy with Win7, way more secure than XP, in my experience.