Windows Vista Home Premium

I installed Vista on my PC it was running XP pro, every thing seems to be
working except I need a co-dec for media player it wont play hadly any vids.
Also my main question; When I boot up after installing Vista a Doss screen
comes up and it asked if I want to boot to Vista or an older version? How can
I stop this and I was under the impression that when I installed Vista it
formated "C" it said it did.
David "Cowboy" Clepper