Put your OS disk in, and reboot your computer. Up in the right-hand corner, there should be a message of what keys to press to enter the boot options menu, usually F8 or F12. Press the key indicated (the "F" keys are in a row at the top of your keyboard) until you get to the boot options menu. Then, select to boot from "CD/DVD drive" or a similar option. A blue screen with a grey bar at the bottom should come up, showing the various files it is loading. After it finishes this, it will show the options to format your drive, and to install windows. First, use the format option on the partition that windows is installed on, (it should be marked as having the windows installation), then select to install windows when it is done formatting.
