"Microsoft Windows 7 is designed to upgrade from older iterations of the operating system, but if you're at all like me, you've been installing and uninstalling apps and utilities, rolling back your system installation after crashes, hacking software packages and DLLs, or even adding and changing registry entries. Sure, you can upgrade your current computer to the new operating system, but it might pay to create a fresh installation and due away with the gunk of years of computing."
I always do a clean install when I go to a new OS. When you upgrade it brings a lot of other things with it that could make the new OS not run the way it was designed. Also if your upgrading because your computer isn't running right, you could have the same problems after you upgrade. This article tells you how and what you need to make your computer ready for a "Clean Installation" of Windows 7.
I for one are looking forward to Windows 7 it is (I think) the best OS Microsoft has built to date. As I have said before I make a living fixing Windows XP.
Try Windows 7 you'll like it!