Installation error

Hi, i have the dvd version of vista ultimate and i already have vista
home premium installed on my computer. Whenever i get to the product
key step on the ultimate vista installation and put in my product key
i receive this error "Installation has encountered an error and needs
to be restarted". Ive restarted my computer and the installation many
times but it comes up with the same error, ive also chosen to not put
my serial key in but it doesnt do anything. please help, i really want
ultimate.