Your best shot is to create a slipstreamed XP Install CD using at least XP SP2. Use NLite to add the manufacturer's drivers for your SATA controller in your laptop. The XP Install routine should now be able to recognize the hard drive controller and install the drivers and let Windows "see" your hard drive.
Before starting, be sure that you have access to all XP drivers necessary to run your PC. Also, make screenshots of the Device list in the Vista install. It can be tough to identify the devices (and find the needed drivers) if you don't have that list.
If it's a Dell (and maybe other big OEM makers) AND if you have Vista Business or Vista Ultimate installed from the factory, then first contact Dell and see if they will help you with the "downgrade" (which would be free).