Here is what I did.
When I got my new computer, I got the full McAfee suite of virus protection. I schedule it to sweep for viruses once a week, usually around 3:00 am on a Tuesday or something. It never finds anything though because it also features a personal firewall that prevents any kind of virus getting through.
I also got Spysweeper. Best $20 I ever spent, the program is easily the best in spyware detection and removal. I never actually get spyware, thanks to spysweeper and mcafee constantly running, but I get the occassional tracking cookie or startup program that spysweeper takes care of. I'd say a spybot/adaware install is just as effective for free, but I'm happy with sweeper. I also sweep for spyware once a week, scheduled at a time i'm not using the pc.
I use firefox
I ALSO schedule bi-weekly defragmentations of my hard drive.
Finally, I'm just careful what porn I download.
So the way I see, I probably won't have to reformat after a year, but I'll still reformat at some point... mainly because I'd like to get a bigger hard drive and partition it off for a multiboot OS system... although I have almost no interest in Linux whatsoever.