The first step to uninstall Vista and install Windows XP is to format the hard drive.
Insert your Windows install disc into your computer's CD/DVD drive and shut down your computer.
When you turn the computer back on, you need to boot from CD. This process is somewhat different depending on your computer. It usually requires you to hit an F-command key when the loading screen pops up. For example, on most Dells you need to hit the F12 key as soon as you see the Dell loading screen. This will boot from CD.
Upon doing so, you should now be on a welcome screen with a blue background and white text. Press enter to go past the welcome screen. The next screen should be the Windows licensing agreement. Push F8 to agree to the terms.
Now, when Windows asks you if you want to repair your existing Windows installation press the ESC key to skip the repair.
You should now be on a screen that lists all of your hard drive partitions. Look for the partition where Vista is installed. More than likely, it will be the largest partition. Navigate to it using the arrow keys and push the D button to delete it. You may have to push the L button to confirm the delete.
No turning back now. Use the arrow keys to select the unpartitioned space. Press the C button on your keyboard, which stands for create. Press enter to confirm the maximum amount of space prompt.
Select the new partition you just created and format it as NFTS. Depending on the size of the partition, this could take quite a bit of time.
After it completes the formatting, follow the Windows XP setup steps. They are very straight forward. It will ask you how you connect to the Internet, your name, password, etc.