However, you need to be careful since some large hard drives are split up into C:\ and D:\ just to make them more manageable. If you aren't sure, it's best to double check before starting the format. You can format a PC without erasing these partitions. You'll just have to select the right drive C:\ to install the new OS.
To start off with, you'll need your original Windows DVD. All of this DVDs are bootable, so once you pop them into your computer, all you have to do is restart your computer and then follow the prompts. If you don't have the DVD, but have a downloaded version of this, it's usually easier. All you need is to click setup and remember your registration number. If the DVD isn't booting for some reason, you'll need to enter the BIOS menu. To do so you'll have to press either F6 to F9 just as the computer boots. It will depend on what kind of a BIOS your are running. Once you are in the BIOS, you'll need to use the arrow keys on your keyboard. The mouse won't work.
1. Press F6-F9 to enter BIOS
2. Select 'Boot' or 'Boot order'
3. Change the order of booting by putting your DVD drive first. You'll have to select a new order, usually a) DVD drive b) hard drive.
4. Press the key to save and exit
5. Restart your computer
This will force your computer to boot from the Windows DVD. All that's left is to follow the prompts and install Windows onto the C:\. This can take between 15 and 45 minutes, depending on how fast your computer is. If all goes to plan, don't forget to change the boot order once your finished.