There was a handy little utility around called ABR that backed up your keys and put them back in post install which I used to use on Vista (not sure if it works on 7 or not, doubt it would do any damage running it though). It was very handy for doing a nice clean install without all the dreadful oem rubbish.
I've checked it out, and there seems to be a version of Windows 7 as well. unfortunately, many people have reported faults with this version. Supposedly because of the difference between Windows versions distributed as OEM already preinstalled on PC made by Dell and such, and home-built PC's with an OEM version bought with it.