Vista installation, failed to boot, not way going back xp or vista

> Today, i was so happy buying windows vista home premium upgrade for
> academic
> only.
> I used windows vista adviser to check whether my computer is eligible of
> installing vista.
> yes it was.
> now, it seems that it doesn't boot as other people mentioned this problem.
> I didn't make any recovery disc, so after installing windows vista and not
> booting, i have no way to go back to xp or vista. I don't know how to
> prove I
> actually had xp in my computer since it doesn't boot.
> when I used start up recovery, it is not working, keep searching for
> problem
> for 5 hours.
> no response at all. i tried safe mode, it is not working.
> I am not professional windows user, so things i have tried were only ways
> i
> could think of.
> I want to know how i can install this upgrade disc without recovery disc
> or
> windows xp disc.( it doesn't come with computer right?)
> as i typed in my product code, it seems that vista license is already
> installed my non-bootable computer.
> it is nightmare.. how can it be so difficult to install?? isn't this most
> famous and top-selling OS in the world?? this should be fixed!!