Well let me put it this way.
As a user you download a lot of applications/games ( not all of them legitimate as I seem to read between the lines ).
Every time you install and then remove those, something is left behind in the windows registry, or something else needed by another application gets accidentally removed and breaks functionality....
Especially in our line of hobbies ( hacking , modding and so on ) we often download applications or cracks that may contain small viruses ( or big ) but we live with the risk and at the end it's worth it.
In order to leave with my choices, I learned over the years to regularly back up my music/video/picture files on an external USB drive, and also keep a folder with all the ZIP files containing full installation files or isos for all the programs/games I intent to keep.
And also I backup my windows mail settings and received/sent messages.
Of course I also keep a backup of my Firefox favorites links and passwords.
In other words, every few months it's advisable to format your hard disk and fresh re-install your OS ( regardless of your choice of system ) to ensure maximum efficiency.
Then you spend a little time reinstalling your stuff and during the process, you realize that you don't even need half of the crap and you end up with a much more clean and garbage free PC experience.
For this reasons, I would never perform an update over a previous OS, because besides all the shortcomings I mentioned before, you also end up with a lot of unnecessary 'left overs' from the old OS that will undoubtedly make your experience with the new OS less then optimal.
I also have Win 7 RC on my system, and after a few months I couldn't imagine going back to XP ( I skipped Vista altogether ) .
Problem is, in a few months RC will expire, and I'm already taking steps to make fresh install of the retail version as easy and painless as it can be.
But now I need a few words of advice myself:
My Dell XPS PC came with Vista ultimate edition with the included Option to have it downgraded to XP.
Which is fine.
I now have a dual booting system with XP and Win 7 RC.
I think soon or later I will have to purchase the retail version of Win 7, and I heard that I won't be able to buy the cheaper Upgrade package and install it over the RC.
But it's possible to upgrade Vista to Win 7 Home for something like $83 which is not as bad as the $ 200 price tag for the full version.
Technically , since my PC came with Vista Ultimate, I should be able to dig out my Dell OS DVD and put Vista on my PC and then upgrade to Win 7 cheaper and hassle free, but my question is, would I be able to perform a "clean" install using the Win 7 upgrade package (by running it from my legitimate copy of Vista of course ) ?
I'm not worried about having to reinstall everything, I'm used to that, is just that a clean install is a must for me.
Maybe I'll start my own topic for this by the way........