If you are trying to install Windows XP to a Laptop where Windows vista is already installed, you will get a blue screen at the Windows Startup in XP installation process. This is happened due to the SATA hard disk.
To overcome this problem you have to change the BIOS settings. First go to the "Advanced" tab in BIOS Settings and you can find the "SATA Controller Mode" at the bottom.
Change the SATA Controller Mode from "AHCI" to "Compatibility". Then Save the Changes.
After that you can install Windows XP.