Is it the disc that came with the computer? If it is, insert the disc and restart the machine. If the computer doesn't boot off the CD, you'll need to change the BIOS settings. When the computer starts, you should see somewhere to press a key for the BIOS, probably F12. If you take too long to press it, you'll need to restart.
Once you get the BIOS open, look around for the boot order settings. Put them in this order:
1. Floppy disk
2. CD-ROM
3. Removable disk
4. Hard disk
5. Network
Save your settings and restart the machine.
Once it boots off the disc, you may get offered a variety of options. Certain OEM discs offer the ability to format the entire hard drive and restore the original image on your computer, which should have all the necessary drivers for the original hardware. Other discs will contain the operating system. As you are reinstalling Windows, just follow the prompts, being sure to delete the current partitions on the hard disk and create a new one. The only problem with the OS disc is you'll also need to use all of the other discs to restore your hardware drivers, which can be very tedious.
