I did a fresh install both times, nothing custom other than doing it via bootcamp.
I forgot to mention it is 64bit.
I did run it through bootcamp with a fresh format each time. I don't see a windows.old folder. I even ran disc cleanup/clean system files command right after installing, and it only had 3mb to remove.
I did notice on MS' website it said it required 20gb with an additional 15gb for winxp mode. I didn't specifically install winxp mode, just ran the Windows 7 install disc and it was all automatic. I assume it is all built into the install by default.
The thing that is puzzling, is that when it installed on a smaller partition, it took up 28gb, and when I redid it on a larger partition, it took up 36gb.