I have recently upgraded to windows 7 from vista.  I do not like to run upgrades and rather do a full install.  With the upgraded version of WIn7 you have to have a product key from a previous version.  There is a fix for this.  If you run the install as a custom install and do not enter the cd key during install you can install Win7 as a full version not an upgrade.  To do this, allow windows to install and prior to activating or entering a cd key go to the root of your system drive and create a Windows.old folder (case sensitive) and then activate.  This will trick Win7 into thinking you have an older version of windows without have the old files on the drive.

Good tip.

I too only do full installs.  In this day and age of nearly unlimited drive space on the cheap it's easy enough to just backup what you need backed up.

Great tip, I did the same thing with the vista upgrade