Here are steps on how you can install xp on pre installed win 7.
On the Setup of your bios.
Choose the ATA instead of AHCI on the SATA configuration..
then now apply the changes. then insert your cd on the cd rom. make sure you have chosen the cd as your first boot device.
Read this FAQ for more informations:
The ultimate solution is by using OEM Windows XP CD which comes with your system, which usually has built-in appropriate driver for S-ATA harddisks running in AHCI mode. However, if you lose or do not have the OEM XP CD, or is given Windows 7 DVD but decide to downgrade to XP, there are 2 options to install XP from scratch.
You can also update the BIOS from HCL official website if they permit it and maybe you'll have the ATA/AHCI option in your BIOS.
Warning: Make sure to obtain the BIOS upgrade software from a reliable source. Downloading a BIOS from sources other than the manufacturer's web site is risky. For example, versions from the same BIOS software developer for one manufacturer's motherboard may not work on a different manufacturer's motherboard. Using the wrong BIOS or version might "brick" the board, thereby requiring either a BIOS replacement or reprogramming of the BIOS by the manufacturer and rendering the computer unable to boot until that is done.
Make sure the electrical power source is reliable when flashing a BIOS. Any sizable fluctuation or loss of power during a flash process can corrupt the BIOS. Therefore, never power down or reset a PC when flashing the BIOS. If flashing the BIOS from a booted OS, first disable all unnecessary applications and background processes.
Avoid updating the BIOS unless you know exactly what you are doing. Failure to follow a manufacturer's directions and precautions may corrupt the BIOS as well Good Luck!
If you don't know where you go, get back where you come.......