I had the same problem. I have a Toshiba Satellite L300 that came with Vista preinstalled. It crashed constantly so I tried to install XP but as you already know, it would say that it couldn't find a hard drive.
What you have to do is power on the Laptop and then press F2 to get into the BIOS menu. Then scroll down to the SATA option and change it from ACHI to Compatibility then exit saving your changes (F10) and then reboot your Laptop from the XP Operating System disk.
It will now let you install Windows XP or any other Operating System.
I too have been searching for answers on various sites and forums and none of the suggestions (including downloading XP drivers) worked.
This definitely works (at least for the Toshiba L300 Laptop anway).