Okay, first things first - do you know how to boot from your CD drive, and do you still have your Windows XP CD?
If so, the first thing to do (assuming you've already backed up anything important on your hard drive, because once you format it, you're going to lose everything) is to boot from your Windows XP CD.
First, you'll get a blue screen with a message saying "Setup is inspecting your computer's hardware configuration". After the hardware inspection phase is complete, you'll get taken to a screen saying something like "Welcome to Setup" followed by a couple of dot points. The first one will tell you to "press ENTER to setup Windows now", which is what you want to do. So press ENTER.
The next screen is the Windows XP Licensing Agreement. Press F8 to agree to it.
The next screen should say "The following list shows the existing partitions
and unpartitioned space on this computer". This is followed by a few dot points telling you to
* To set up Windows on the selected item, press ENTER
* To create a partition in the unpartitioned space, press C
* To delete the selected partition, press D
Following that is a box listing your existing partitions. You can highlight (i.e. select) different partitions using the up and down arrow keys.
If you're wanting to change your partition sizes, what you want to do is first delete the existing partitions. So select the first partition and press D.
If this was your previous system partition, you'll get another screen with a dire warning to the effect of "The partition you are trying to delete is a system partition, BLAH BLAH BLAH". As long as you're comfortable that you've backed up everything that you need, press ENTER to delete the partition.
Next, you'll get a screen saying something like "You asked setup to delete the partition" followed by the partition you chose to delete. Press L to confirm the deletion.
After this you go back to the screen which lists your partitions. There should be an item listed as "Unpartitioned space", as well as any partitions you have not yet deleted. Repeat the above steps to delete the remaining partitions (if there are any - if you only had a C: partition before, then you should only have unpartitioned space left in the list).
Now, the only thing left in the list should be "Unpartitioned Space", with a listed size equal to the size of your hard drive (the actual size, which will be a couple of GB less than 80 GB). So now you want to create your new system partition - press C.
You will be prompted to enter in the desired size of the partition in MB. With an 80GB drive, you'll probably want to make it around 10GB at least. I prefer to use about 15GB or so, since I tend to use my Desktop as a temporary holding space for stuff, and you can run out of space quickly. But make it as big or small as you want (probably not smaller than 3GB though).
The next screen will say something like "A new partition for Windows XP has been created on BLAH BLAH BLAH" followed by four formatting options. Choose "Format the partition using the NTFS file system". Don't choose the "quick format" option - the non-quick format checks your disk for bad sectors, which are flagged so that Windows doesn't put any vital system files (or any other files for that matter) on a damaged sector.
Next, you'll get a screen saying "Please wait while Setup formats the partition...". Once the formatting is complete, you'll get another screen saying "Please wait while Setup copies files BLAH BLAH". Once this has completed, you'll get another screen saying "Please wait while Setup initializes your Windows XP configuration".
After this, you'll get another screen saying "This portion of Setup has been completed successfully", followed by "To restart your computer, press ENTER". There will be a red progress bar as setup waits 15 seconds or so, then it'll reboot itself if you haven't yet pressed ENTER.
At this point you should go into your BIOS and change the boot order again. It's not absolutely essential though, because if you leave it for long enough without pressing a key, it'll start to boot from the hard drive anyway.
After this you'll get to the Windows GUI setup, which is pretty easy to follow. Once that has completed, you'll have your new system ready to go. It would be a good idea to install your motherboard drivers before you install anything else. Then install your video card drivers, set the resolution and refresh rate to your liking, and install any other necessary drivers.
Once you've reached this stage, you can set up your remaining partitions by right-clicking on "My Computer" and selecting "Management". In the Computer Management window, look down the left pane for "Disk Management", which will appear under the heading "Storage". You may need to click on "Storage" to expand it so you can see the "Disk Management" option.
You can always ask again if you don't know how to use Disk Management to set up your partitions.
