Sorry about that. Here's a link to the GParted website; you can find a full explanation there. In a nutshell, GParted is a Linux-based program that runs from a CD or USB drive that lets you edit the partitions on your hard drive. You can resize, move, delete, create, and reformat partitions; it's a powerful tool! It's popular as well, so if something is not explained well enough in the official documentation, chances are a Google search will provide you with more help and info.
1. Go into your bios and set your boot order from the hdd to the dvd drive 1st
2.Reboot with your windows cd in the drive.
3.you will be prompted to press any key to boot from cd
4.select complete installation from the options presented
5.select your primary partition and click format (fast format)
6.Continue with the installation.
You will lose ALL data the second you format the partition.
If you computer has a reserved partition with windows on that do not format it.
(if your computer came without a windows cd there is a good chance you have a system partition.)
In this case google how to reformat (your model here).
Make sure you have all relevant motherboard drivers beforehand or a way to access them with your other computer down, without those drivers you will have no internet, no sound etc etc.
If you have a cd then you can go ahead with my instructions up top. Once the cd boots its pretty self explanatory, just follow the prompts.
My warning above was based on the fact that a lot of prebuilt sytems and laptops have there copy of windows actually on the hard drive in a seperate partition.
Deleting this partition would delete your copy of windows.
Since you have a cd in your hand the above warning does not apply to your system and you can reformat your partition.
Formatting of the partition will occur as part of the fresh install when you boot from the cd.
1.Set your boot order to dvd 1st in your bios.
2.Restart the computer with the cd in the drive and press a key when it asks to boot from cd.
3. Follow the prompts menus.
tonkatuffmofo's suggestion will certainly work as well (having Windows format during install). But are you sure you have that Windows installation CD? Since your PC was prebuilt, any recovery discs you can burn using HP's recovery manager will not allow you to format. That's why I suggested GParted. Anyway, the utility that formats a USB drive to run GParted may be found here.