Hit the del button after you turn it on so you can get into the bios. After you're in the bios find "load optimised defaults" or something of that nature. hit F10 and then Enter ( this will save it ). The computer will reboot. Hit the del again to get back into the bios. once there look for boot priority or boot sequence. You want to make the cd-rom or dvd-rom the first boot device, the HD second and if you have a floppy make it third. If no floppy... disable it. Put in your operating system DVD. hit F10 and then Enter again. machine will reboot. Let it do it's thing until it wants you to "hit any key to boot to cd-rom". This will begin the install of the opsys. Do what it says on the screen. be patient. if you screw up the first time you can do it again.
Once it loads the opsys it will start to reboot again. This time DO NOT hit the "hit any key to boot to cd"....... just let it go to the desk top and finish the installation.