VISTA PROBLEM

Hi I am another one of the croud who tried to download windows vista sp1 - now it won't boot properly.
I have tried using the Dell re-installation DVD but that won't work - it loads files and then I get a message telling me that a problem has been detected and that windows has been shut down to prevent damage to my computer
I have also tried Spotmau power suite download but it won't boot properly
Whatever I try I cannot get to a Windows recovery window option
I feel that I would like to change my operating system to XP, and I have another computer which uses XP. If I try to install it over the Vista it starts to instal but when it asks me to press F8 to agree to conditions, the F8 key won't work - so I have to escape from the installation.
Is it worth while getting a Vista CD - and try to re-install, or should I try buying an XP Cd and try to install that over Vista - I think that it won't install unless I re-format the Hard drive. If that is the case, how do I re-format without being able to boot and run Windows?
Lots of questions I'm afraid, but really all I want to do is start using the computer again - I don't care if it's XP or Vista