I never format the whole hard disk, just format the System partition of windows once a half year.Virus infected the system file could harm a pc, format the system partion is enough to remove them totally. I think it's useless to format all partitions.
My PC is 3 years old but still running OK without formatting once. Just regularly run system maintenance tools and keep your antivirus software updated.
I format my C drive almost every month, I have created a backup using (Norton Ghost), after installing windows and all the softwares i use often and changing their settings, and moving my program files, and user **** on another drive(D:) now whenever there is any problem, I just restore from that backup, as i hav saved settings and programs on another drive, i dont have to copy stuff or worry about settings, etc when restoring, and it takes only 10 mins, to start using windows as if nothing happened.
I did't format my hard drive freq. as it could be damage it.
Will format it when it come will lost clusters but can not be
recovered by some hard drive utiltiy.
I have my drive used almost 6 years but haven't re-formated it.
And all my system works without any issues.