You'd remove Vista and install XP.
First talk to the manufacturer to see if they offer a free or lost cost "downgrade".
Second check that your particular laptop does have a package of drivers for XP - some don't. Download them and put onto a CD.
Third - buy (if you can't get free) a copy of XP - better rush though, not so many around now.
Fourth - Save all your data and settings.
Fifth - Install XP - formatting the drive in the process to remove Vista.