Before you go down this path there are a few things to check. If you can't get XP drivers you may as well forget the project.
If it is a brand name PC or NoteBook check the manufacturers site for XP drivers especially the Chipset, Video and Network. With Vista preinstalled some manufacturers will supply an XP Recovery CD. If you have access to that CD you can boot from it and it will install the Operating System and set it up for you. You wont need the rest of these instructions.
If it is an IBM clone you will need to find out the Motherboard ID to be able to source the drivers. Download CPU-Z and extract it to a folder. Run cpuz.exe and click on About and under Tools you will see Registers Dump (.txt) click on it and save the file. You will be looking for Mainboard Model and you can Google it to find the drivers for it. You may also need to slipstream Sata drivers into your XP install CD. See instructions below.
http://www.cpuid.com/cpuz.php
When you have the necessary drivers and your XP CD you can proceed with the installation. Copy your Drivers and Antivirus software to a CD or a USB Stick. Check in the BIOS to make sure that the Boot sequence is set to Boot from the CD/DVD first. One of these keys should get you in.
Common BIOS entry Keys
F1
F2
DEL
ESC
F10
You will know when to press this key when they see a message similar to the below example as the computer is booting. Some older computers may also display a flashing block to indicate when to press the F1 or F2 keys.
Press to enter BIOS setup
Tip: Hold a Key down, this will cause a stuck key error, which may allow you to enter the BIOS setup.
You will have to remove the Vista Primary Partition and recreate a Primary Partition before you can install XP.
Perform a clean install of Windows XP and delete the Primary Partition and recreate the Primary Partition on the fly.
1. Start your computer from the Windows XP CD. To do this, insert the Windows XP CD into your CD drive or DVD drive, and then restart your computer.
2. When you see the "Press any key to boot from CD" message, press any key to start the computer from the Windows XP CD.
3. At the Welcome to Setup screen, press ENTER to start Windows XP Setup.
4. Read the Microsoft Software License Terms, and then press F8.
5. Follow the instructions on the screen to select and format a partition where you want to install Windows XP. At this stage you can select to delete the Primary Partition C: Partition1 and recreate it. Then continue with the install.
6. Follow the instructions on the screen to complete Windows XP Setup. Install the Motherboard drivers and your Antivirus. It is a good idea to have a copy of your Antivirus to be able to install it before going on the internet.
http://support.microsoft.com/kb/316941/en-us
Good Luck
How To: Slipstream your XP installation
Download nLite
http://home.midmaine.com/~nlite/nlite/nLite-1.4.8.installer.exe
These are the steps that I took
Download the Sata Drivers from the manufactures web site.
Installed Nlite
Extracted Sata Drivers to a folder named Sata
Copied the entire XP CD to a folder named XPS
Ran Nlite and Browsed to the XPS folder, Next, next then selected Drivers and Bootable ISO click next
At the Drivers Tab click insert, single driver and navigate to the folder with the inf for windows XP.
Highlight it and select open and select next. Select Textmode driver and select the XP Sata driver and click OK.
Click next and Yes to apply changes, next and select Make ISO. Use Nero or similar to burn it to a CD.
