Summarize the following:
If possible, you will want to backup your data before wiping the computer, as all data will be completely erased. You will need a Windows 7 installation disk or alternative OS (such as Linux, which is free) as the OS will also be wiped and you will need a new one if you wish to use the computer again. You will need to download a program for securely wiping your computer. DBAN is a common program but there are also others, like Active@KillDisk. Be sure to download the program from a reputable source. You will need the program on some kind of storage device from which your computer can load. This should occur automatically. If it doesn't, you may need to enable this in your BIOS settings. Start the computer and as it is loading press F10 or F12 (depending on your system and keyboard) to access the Boot menu. From there you can specify to boot from a USB or CD first. Turn the computer off and when you reboot it should start from the specified device. Once the computer boots from the disk, the fastest method to wipe the computer will be to type in "dodshort". You can also open the interactive mode if you want more options. Hit enter and it will start wiping. Once it is done, reinstall your operating system and your computer should once again be functional. You will only need to restore files from your backup.
Backup all of your data. Have a replacement OS. Download Darik's Boot and Nuke (DBAN). Put the program on a CD or USB. Plug in or insert the wipe program disk or USB. Boot the computer from the disk or USB. Type "dodshort". Hit enter. Reinstall your operating system.