I believe what DaChew is subtly trying to ask is do you have a CD for an operating system with a valid product code. If you wipe the drive you will loose all the information that's on it.
If you want to wipe the drive and have everything that you want to save backed up you will need a utility like Killdisk
to wipe the hdd. This is a free utility, download this and transfer this to a CD, place this in you CD-ROM and reboot the computer and follow the instructions, one over writing will be sufficient for your purposes. Now you are ready to do the same procedure with the Windows installation CD.