If your computer has a "designed for Vista" or a "designed for Windows 7" label, and you install Windows XP, your results will be unsatisfactory; Windows XP will not have device-drivers for many of the components (audio/video/network/camera/microphone), although Windows Vista _will_ have the correct device-drivers.

Stay with Windows Vista, or upgrade to Windows 7.

Or, you might try:

http://www.laptops-drivers.com/compaq/how-to-install-windows-xp-into-compaq-cq40-101tu.html

Good luck -- you'll need it.

Initially you have to download sata driver for the model of your laptop from the manufacture website.
After downloading driver you have use the nlite software to create xp cd boo table with sata driver.
you can get details regarding the creation of bootable xp with sata driver from the following link
http://news.softpedia.com/news/Install-Windows-XP-On-SATA-Without-a-Floppy-F6-47807.shtml

you can download software from http://www.nliteos.com/download.html