Insert the Windows installation disk into your CD drive. Shut down your PC. Then, boot from CD.
This part is important: do NOT run the Windows installation from Windows itself. Shut down first, and then boot up the machine from CD. My Dell has a little message as it's booting up that says "Press F12 to boot from CD," so that's what I did. If you're not sure how to boot from CD, check your PC's user guide for more info.
The reason why booting from CD is important is because we're going to delete the C: drive partition where Windows is installed and re-format it. You can't do this if Windows booted from C:, because it can't delete the partition from which it is running.
You CAN install Windows without deleting the partition and formatting, but that means all your program files and other riff-raff that's collected on your C: drive will still be there when you're done, just taking up unnecessary space. That's not the point of all this. Be sure to boot from CD.
Step through the Windows installation.
You'll be greeted by WordPerfect 5.1-like blue screens with white text on them, which seem scary, but aren't. All the directions are clearly spelled out on each of them. Still, we'll go over what to do.
At the Welcome to Setup page, press Enter. Press F8 to accept the Windows XP Licensing Agreement. You'll be asked if you want to repair your existing Windows XP installation. Press ESC to bypass the repair and install a fresh copy. All your existing disk partitions will be listed, like this.
You want to delete the current partition where Windows is installed. Use the arrow key to select it, and press D to delete it. Press L to confirm. Then, to create a new partition, select the unpartitioned space and press C. To create a new partition with the maximum amount of space allotted to it, press Enter.
Now select the brand spanking new partition you've just created to install Windows on. Format the drive as NTFS (Quick if you want, but I went thorough just to be sure.) Depending on the size of the drive and how fast your computer is, this will take some time. Get a sandwich. Then, follow Windows Setup's steps, set your area code and name and password and let it reboot as many times as necessary until it asks you to log in for the first time. Congratulations! Welcome to your fresh new Windows installation.