First and fore most make sure the key you have will work with the copy of the OS you plan on installing. Next enter the BIOS and have the system boot to the CD/DVD drive first. When you restart the system it will say press ENTER or PRESS ANY KEY if you want to boot from the CD/DVD. Once there make sure to remove the partition (this will show and you push D then L then something else I forget. Then create a new partition and install your OS to on that partition.
WARNING: make sure you get all the information from the DEVICE manager on whats installed and make note of the MB model number so you can get the Chipset drivers, Lan drive (if it has onboard Lan, Sound etc etc,
My $1.00 / $2.50 worth
TD