Step-by-step guide
Important: If you use a wireless adapter to connect to the internet, make sure you have any drivers you need to make that work backed up on a USB flash drive or something. You don’t have to worry about any other drivers right now, as they all can be accessed later once you get online.
Step 1: Booting from Disc
If you’ve never done this before, you need to know a couple of things about how to boot from the Windows DVD. First, put the disc in the drive and then restart your computer. You need to be quick with this next step, since there is a period of about 2-5 seconds in which you’re going to interrupt the normal boot-up process of the system. As soon as the monitor comes on and displays the BIOS information or manufacturer logo, you have to press a key specified at the bottom of the screen to access the Boot Menu (mine is ESC). From there, select your disc drive from the list and the computer will boot that way this one time. It may require you press a key, so do so when prompted.
If you have an older motherboard/computer, you may not have a boot menu, in which case you’ll have to set the boot order manually by accessing the BIOS settings. That will be done by pressing a different key (usually DEL or F2) and bringing up a screen with a blue background like the one below. Look around in the different menus and find the “Boot Priority” or “Boot Order” option (a user’s guide for the motherboard/computer in question should help if you’re lost). Set your disc drive to be the primary boot device, as shown here:

Don’t mess with any other settings if you don’t know what you’re doing! Back out to the main menu and then select the option to save settings and exit. Your computer will restart, and this time it should check your disc drive and give you the option to boot from the DVD. Press a key when prompted.
Don’t forget to come back later and set the primary boot device back to your hard drive.
Step 2: Windows Setup
Now Windows should be loading from the disc, which can take a few minutes. Once it’s done, you should see a fancy blue background with the Windows logo in a dialogue window. Click “Next” to use the default language settings (unless you want to change those), click “Install now,” check the box to accept the license terms, and then click “Next” again. Since you’re not upgrading from a previous version of Windows, you’re going to do a “Custom (advanced)” installation, so click that button. This will bring up the list of hard disks you have in your computer and the question “Where do you want to install Windows?” Finally, click “Drive options (advanced).” Now is the time to decide whether or not you want to mess with partitions (Step 3) or formatting (Step 4). If not, skip to installation (Step 5).

Step 3: Partitions
A partition allows you to section off virtual disk space within the physical hard drive you have in your computer. In other words, you can make Windows think that you have multiple hard drives when you really don’t, by formatting the disk a certain way. You can split your 500GB hard drive into two 250GB ones, five 100GB ones, whatever you want. Think of it like using a curtain or cubicle wall to separate different areas within the same room. You can have your work desk in one area, couch and a TV in another, etc. People partition their hard drives to make it easy to organize files (Windows installation on this one, programs installed on that one, games installed here, backup data stored there…), ration disk space out to individual users, or you can even have multiple operating systems on the same hard drive and select which one to load whenever you start up your computer!
If you want to do this, you’ll first need to delete your old partition, which probably occupies your entire hard drive. Some computer manufacturers will create a back-up partition in lieu of giving you a system restore disc, so that could explain an extra partition you have that you don’t remember creating. Also, Windows automatically creates a small (100MB) partition labeled “System Reserved,” in which to store setup files, so don’t worry about that if you see it. Make sure you’ve already backed up any files you wanted to save! Click on the partition you want to reclaim the disk space from (probably the one that has Windows already installed, then click “Delete.”
Essentially, you’ve just formatted your hard drive, deleting all the files that were on that partition, and you should see a listing for “Unallocated Space.” Click on that, and then click “New” to create a new partition. The number in the box that shows up is the maximum space you can allocate to this new partition; you can leave it at the default to create one big partition, or divide it up any way you want. Once you have your partition(s), you’re ready to install Windows, so move on to Step 5.