Saab,
Interesting about the Dos v5.5 - even looking on wikipedia it says it does not exist yet I am sure the tulip dealership I used to work for had it - maybe it's me getting old (it was 15+ years ago) or was a tulip specific thing so I concede your point there.
BOAC:
Have you looked through
How to uninstall Windows Vista service packs as a troubleshooting step as it gives advice on unistalling a vista SP from a command prompt (Scroll down to method 3) . I think it requires you to have booted into vista in the first place but it may be worth playing with some of the advice on that before trashing and starting again.