This question is a rather frequent one appearing in question and answers sites. Ordinary people would easily think about a hard disk needing to be reformatted from time to time as they tend to get such advice from who they consider experts in computers whenever their computers get so crumpled with junk software, adware or even spyware and viruses.

Does it really need to be done?

This is a better question. The need to reformat a drive is never there, unless we want to purge all of the hard drive's content, and we really do not want to waste any time saving or backing up data to another drive. Even if we do, the time it will take reformatting a drive is incomparable to just deleting certain folders, performing a thorough disk check, and then just reinstalling the operating system and all software we need.

So it does not really need to be done after all. We just have to in the worst case delete certain folders and reinstall the operating system, but not reformat the entire drive. Doing so would be like plowing the whole backyard in order to plant a single flower in the middle.

But for the sake of answering, here is the answer when a hard drive needs to be reformatted or at least an operating system reinstalled.

1. It depends on how many times we perform software installations and uninstallations, successful or not.
2. It depends on how many times we contract viruses and cleaned by antivirus. Unsuccessful cleanups accounts for even more system crumpling.
3. It depends on how well software we installed is written. Badly written and buggy software surely adds up pretty much to the need to reinstall or reformat a hard drive.
4. It depends on how many times we surf the internet and are not aware that we actually installed a software or worse yet – malicious software. The latter could be just spell the end of the computer's life.
5. It depends on whether or not we have sensitive data on the drive that we need to purge completely. This one needs us to either reformat the drive or employ software to delete thoroughly deleted files.

Any computer system would have its own characteristics as to how the users of that computer behave in browsing the internet, downloading and running files, what programs are installed and run, how the users would install new programs, the list is very long.

A very savvy user being the only one who uses a computer would end up never needing to reinstall the operating system or even reformatting the drive. On the contrary, an average to less knowledgeable user using any computer for an extended period of time  or a computer being used by a number of different users with varying degree of knowledge about computers would render the computer very complex in system and traces just because they do not know how the computer works internally and programmatically.

The main thing in this case is to keep the computer safe, and to realize that reformatting is almost never needed. Instead, an operating system reinstall is the last resort. Also what we need to remember is that we can learn how to use a computer the right way in order to prolong the system's life time before the next reinstall or reformat.

Notes:

Installed antivirus is another thing you might want on your computer to prolong the need to reinstall the operating system. You may refer to an antivirus review for the best one at Top Ten Antivirus 2011 Review.