The process of Computer Format is not as hard as everybody thinks. Depending on what you want to do, you can find some great resources there at How to Format a Computer.
What you need to know is before you decide to do computer format, is to backup some of your files located in the local hard disk where your current operating system is installed.
In details: Before you do a computer format – backup all the files in My Documents (ex. messenger received files, your personal documents etc) basically everything that you may need from the My Documents folder because that folder will be completely deleted. Next, take a look at your desktop. Do you see some documents you may need ? Back them up too (everything except shortcuts). After you’ve finished backing up My Documents and Desktop, next thing to check is the C: drive (or other drive, if C is not the drive where you’ve installed Microsoft Windows).
Now after you’ve backed up the documents you need, depending on the operating system you are going to install, proceed to: