I've had the exact problem before!
Upon boot up my computer just simply was stuck on the screen with the bar loading then goes blank and never loads up the Vista sign on page.
I couldnt boot to either safe mode or normal mode either because it would just get stuck.
It means that a file on your windows was corrupted.
The only way around it is a system reformat back to factory new condition
The reason it happened to me is because of a Vista update that wasn't allowed to fully complete it's installation upon reboot (my battery died and therefore cut off the update in progress).
Maybe you have a different incidence but same basic outcome.
However, i'm having a bunch more problems with Vista and I've decided it's most likely my HDrive.