First back up all the files you wish to keep.
If it is an OS X your installing. do this.
Put the installer disk into the DVD Drive, Hold down the C key and start your computer, now, move your mouse cursor towards the top of the screen and the OS X menu bar will appear. You should see a drop down list for "Utilities". Click "Utilities" and select "Disk Utility". Disk utility should now be opened and visible on your screen. You will see your hard drive listed along the left side of the disk utility window. Click it once to select it. Next, click the "Erase" tab located in the center of the disk utility window select the volume Formant, Mac OS Extended Journaled, give your hard drive a name. You will see three options: erase free space, security options, and erase. If you want to erase your hard drive securely with no chance of data recovery, click security options and select Zero out disks. If you just want to erase your hard drive and install OS X, click erase. The "Erase" button is the much faster option.
Quitting Disk utilities will take you back to the installer disk, select your newly named hard disk and install your chosen OS X system.