This might not be that easy if you think about it. You have to know if you want to still have Vista on your computer by setting up a dual boot or just wipe out Vista and load Windows XP.
If your going to do a dual boot on your computer to have Vista & XP installed download the Disk Part Tool on the Vista installation disk or GParted to create a second partition.
Disk Part Tool
DeParted
You can also use EasyBCD - you'll have to add the drive letter and change type to Windows NT/2k/Xp/2k3. Click add entry then save. When you'll reboot your computer the bootloader will appear.
EasyBCD
Note
Be aware that some high performance computers can't handle AHCI mode on SATA drives so you would most likely have to go into the bios and disable AHCI mode.
Also when doing a dual boot with XP & Vista you'll have problems loading Vista. That can be fixed by using the Vista DVD and selecting Repair your computer ont the install screen and select start up.
