Hi Christian,
You will need to obtain the XP drivers for your hard drive controller.
These should be available on the Dell website. They will need to be on a
floppy disk. Follow the instructions for creating the floppy. Insert the
floppy disk when you see the message at the bottom of the install screen
"Press F6 to load...drivers". When you get to the stage of choosing hard
disk/partition to install to, you will need to delete the partition, then
create and format a new partition on the disk.
What model is your laptop? Please be aware that some manufacturers are not
being forthcoming with XP drivers for some of their recent models, so do
check first. Also note that the Dell warranty will likely not cover you in
the case of serious problems. They would expect the laptop to be in the
state that you received it, ie with Vista installed, before assisting you.
May I ask what it is about Vista that you don't like?