installing xp over vista

I’m having a problem with my toshiba u300 152 laptop.Somehow i managed to forget my password or someone changed it(use it with my cheeky brothers).I now cant log into the machine anyhow
and i dont have the vista cd for reinstallation.I’ve been trying to load xp but whenever it starts the installation process it reaches a point just after copying the boot files it cant find the harddisk installed.
Am stuck from there,i tried the repair option so that i can remove vista with the dos prompt but when i click on repair it says it cant find a hard disk.
Anyone with an idea as to how i will go through with this????