I cannot find an "About..." link or similar in the release version of Ubuntu One for Windows. How can I tell what version number of the client I have currently installed?
Open a terminal and type
Navigate to the Windows Control Panel and click the item that is—in various versions—referred to as "Add/Remove Programs" or "Uninstall a Program". You will find a list of installed software and metadata about it, including the version number:
On some versions of XP you might need to click the "Click here for support information" link for the Ubuntu One entry to reveal the version number.