Take an old (or new, 1TB drives are under $99 nowadays) hard drive and install 7 on that... That way you have access to all your files and won't lose anything if you screw it up.
I did it that way and transitioned over slowly... When I remembered something I needed I moved it from the XP drive to the 7 drive, or would boot into XP to run something that wasn't installed on the 7 system, then install it on 7 later... Now I find myself needing the XP drive less and less.