I don't really format any of my machines unless an OS breaks beyond repair or I install a new Linux distro.
Back when I used Ubuntu more, I did a clean install for every new version (6 months cycle), but later switched to a rolling release distro, so no reinstalls are needed. I don't remember ever having had to reformat any of my Windows boxes. (I'm not sure how people catch so many viruses, but I don't pirate and generally don't download a lot of executable stuff.)
Windows 7 has only been on my desktop for two weeks, so can't say much about it. I can say, though, that if I had to reformat more than, say, once a year, I'd switch platforms. (I have five machines: one with W7, one with XP, two with Linux, one with FreeBSD.) I mean, it's fine for an experimental box, but anything I do work on really needs to be stable and mature enough to not require me to reformat the HDDs.