Are you saying that your laptop came with Vista pre-installed, or did you get it with XP and free express upgrade to Vista?
If the answer is the former, then no, you cannot. You would need to purchase a copy of XP (probably only available now online or Ebay, re-format your drive and install XP cleanly.
For the free express upgrade, I'm not 100% sure of the answer. You may want to call Tech Support of your computer manufacturer.
Hope this helps?