Forgive me, if to a large extent, I may be adding to what has already been said...
A bit of 'history' b4 I sum up...
I have lost count of all the installs; between Beat Testing Vista (Builds), upgrading & expanding my machine; virtual machines, multiple-boots, migrating from Beta to RC to...
Never mind past scenarios, currently I have Vista, Win7 32 & Win7 64, all non-virtual, plus VMs of XP (mostly now for client support or the odd thing that stops @ XP) & Vista (this for beta testing).
(BTW, I had put, as a Beta Tester, the same query to the Win7 Build Team, "could I 'Upgrade' Beta to RC?"; they gave me a 'workaround' which, ultimately, I did not use. @ the sametime they still recommended a Clean Install which, is what I did.)
Point being, I have done Clean Installs over & over.
Do a Clean Install... end of story. It is a wee bit of 'work'?, sure; is it or need it be a burdensome arduous pain?, NO.
Be (pre)organized & methodical.
The ONLY thing the need be on the OS drive is the OS. Sure, things like Office or whatever, maybe, but, it is quick & easy to reinstall such things, anyway.
Everything else you have stored or sitting @ the ready on another drive. The beauty of it is THAT drive is accessable from the OS. All or any Tools, Utilities, Installers you don't lose/haven't lost AND you just rapidly put things in or back from there.
Clean is best & can be smooth & pleasant.
Regards,
Drew