Vista requires at least 20GB for install. If you dont have a DVD Burner what you need to do is use Daemon tools or something like that and extract the ISO to the 5 GB partition. From there run the setup from within XP. It will work then.
My piece of advice dont think you can use Vista as a main OS. I have tried and found myself back in XP many times. Nero doesnt work right, let alone anti-virus apps. No firewall apps isntall. Other software for overclockig doesnt work. Firefox and Opera are both buggy in Vista. So there are many bugs to be ironed out before you can use Vista as a main OS. Hence the term Beta.