Why you might want to reformat your hard drive.
Over the years we tend to download and install a myriad of things into our computers. We install and uninstall many programs, create and delete many documents, images, music files, etc. And with all the Internet surfing we do there usually comes a time when our computer does not act or perform as well as it did when it was new and pristine. It slows down. Some things don't seem to work right. If it is not a critical issue we tend to work around it, or just ignore the little corruption that is occurring. But what often happens is that the corruption gets to the point where we can no longer ignore it, or at its worst we face the 'Blue screen of death' – meaning the total failure of the operating system to do its job.
At this point we are forced to reinstall the operating system. To do a clean reinstall of the OS it is necessary to reformat the partition on the hard drive where the OS will be reinstalled. This brings up the reason I always recommend having more than one partition on your hard drive. If your data (documents, photos, music, etc) is stored on a separate partition or separate hard drive than your operating system, then your data is safe when having to reinstall..
After reformatting the hard drive and reinstalling the OS, the system drivers (Motherboard, Video Card, Sound Card, Network Card, USB etc) have to be reinstalled, then the drivers for your peripheral equipment (printers, scanners, etc). Then your OS updates have to be downloaded and installed. Once that is done then all of your programs have to be reinstalled and updated. It's a big job, but once completed you have a computer that will perform like new again.
You can see why it is important to keep all installation disks handy for any programs that you may need to reinstall in the future. I would also like to note that if your operating system is Microsoft it is important to have an authentic Microsoft operating system on your computer. If you do not, then Microsoft will not allow updates to your computer. Without the updates your operating system will not perform correctly.
Now that you know why you may want to reformat your hard drive and reinstall the OS - I suggest that you don't try this yourself if you have never done it before. As mentioned, it is a big job and the sequence of events need to be performed in a certain order for the final result to come out as intended.