Following this, it helps to goto %AppData% with Windows Explorer and to locate the Zimbra folder. Therein you can see three subfolders, at least on my Windows 7 Professional x64 installation. Moving or deleting those folders away will reset your user Zimbra settings. At a new start Zimbra Desktop will launch and has to be re-configured.
I assume that this behaviour is caused by having the users' data outside the standard path C:\Users in this case in D:\Benutzer\USER\... (which is German for Users).
I hope this helps you folks to solve or at least work around the issue.