Were you at any point previously able to log in?
If not try to use Last Known Good Configuration. It will load the system as it was when you last sucessfully logged on.
If that does not work, if you have any important data then first back up everything you need if you are able to (should be backed up anyway in case something like this happens). As you can't boot up, take the hard drive out and put it into another computer and copy the files over.
Once you have all important data backed up, reboot your computer and try hitting each F key in turn (F1, F2, F3 etc). One should enter the recovery partition. This will allow you to take the system back to the factory settings, exactly as the computer was when you first bought it - so Vista, the bloatware that comes as standard and nothing else