First you need to be sure you want to delete really everything because after that it you wont be able to recover it. If you are sure you need to find OS that can be booted from CD. Today every OS can be booted from CD. If you are using Windows on you computer you will need to find your installation CD. It doesn't matter if it is the one you installed your current OS from it just has to be boot-able. If you don't have have any CD with OS on it you can download any free distribution of Linux just for this. You can then burn it on CD or use special program to make your USB boot-able with this OS. When you are ready you need to go to your BIOS by pressing some keys on BIOS boot screen (usually F2, F9, F12 or Del), find boot priority and set it to CD, USB according to what device you are going to use. When all is set put your USB, CD into right place and restart PC. You should see your PC booting into system on your device. Here you need to find disk/partition manager and format the whole HDD. If you cant find it you need to select install and follow the steps until prompted to select HDD particle to install. Then you select format the HDD and wait. After that OS will probably try to install so if you don't want it the stop it. When done your HDD will be empty. Everything will be deleted and you will have to install OS and everything else on it again. Some Operating Systems may have ability to run from so called "Live CD" which is basically the same as running OS from HDD but much slower. Ubuntu and other "buntu" Linux distributions have this ability but it is not recommended to use this every day as you cant really save your work there unless you are using USB and its really slow.