As others have said, a clean install is the surest way to get off to a nice clean rock solid start, however, if you have a lot of programs and settings to transfer, that can be a lot of work even with Windows Easy Transfer. I personally am of the opinion that the upgrade process in Windows 7 seems to have been improved quite a bit. You might consider trying an upgrade install from Vista and seeing how it works out. If it doesn't work to your satisfaction or has issues, there is nothing stopping you from then doing it over as a clean install. So nothing risked by giving it a try.
Just make sure, whatever you do, you have a good complete backup of your original data or preferably the entire system before you start upgrading/installing, ideally, to external media.
