If you've downloaded the RC2, since it's what i use, you won't lose anything at all.
I mounted it with daemon tools and it installed fine. Made a backup of my old windows to boot.
You NEED at least 20 gigs of free space, depending on how big your current windows folder is. Vista requires at least 12 gigs.
Your license key is good for a year, but i'm only keeping mine installed for a month or two.
My only current problems is a bar in explorer i can't get rid of, a horrible start button(it's so bad[so bad]) and Sexy Beach 3 didn't work. If there are pics anyone wants, pm me and i'll post them here
Jam it back in, in the dark.