I format... never, if I don't have to. I haven't been forced to yet, there's almost always a way around it. Bad virii? Meh, haven't had a virus before either. Although my NEW HD did crash 6 months after purchase... I was using a trusty Western Digital 8.4 Gig before... then upgraded to a 20 Gig fugitsu and it crashes...
Before that I hadn't formatted... well for 2 years and 6 months (Around the time I got my first computer :)) I've had my share of problems though, but I guess that's how you learn.
I'm running a Duron 800, 192MB of RAM, 8.4GIG HD, and an old nVidia TNT video card on Windows 2000. Friends of mine who think they know a lot about computers are constantly reformatting (like 4 months or so) even after that, they blame it on something beyond their control.
Formatting is the universal solvent, (next to water of course) it cures all problems with little or no thought.