I have always been told to format once every six months. So that is what we do.

I use Norton Internet Security, Spybot Search & Destroy, AdAware and make sure all are regularly updated. Set Windows Update and Norton to update automatically. Run Windows Disk Clean-up and Defrag regularly and I haven't had any major problems.
We do all of those things as well. Like I said I have always been told every six months. I wasn't meaning for only major problems. We have to do it every other month or so because of certain issues we have with our computer, that aren't really fixable with out a format.