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The installer program for Revit Architecture 2011 crashed during my initial installation attempt. The web site now shows that I have installed the software. I did get far enough to receive a serial number and product key for activation but have no software. Now I cannot download the software again because the site thinks I already have it.
you can download the trial version and install it with your serial number (source is the same)
you could try to clear browser cache and cookies to fix site issues.