I am currently runnng Calibre x64 2.0 without problems.
I have downloaded and attempted to install v2.1 without success. Initially, I received the following message:
The specified account already exists
After several attempts, including a reboot along the way, I googled the message, and it was suggested that I try the Microsoft Fixit Program InstallUninstall procedure.
I did this and it seemed to be successful. It certainly claimed to have fixed a problem, but at the conclusion of the process, which included an uninstall of Calibre and run of the 2.1 install, Calibre was fired up and I still found myself running v2.0!
Shades of Alice through the Looking Glass! Perhaps if I try to install v2.0, I shall find myself with v2.1.
But seriously, could it be that there is a problem with the intall package?