Have you tried manually restarting Yahoo! Zimbra Desktop service and look for any errors in Event Viewer->System ?
Ok this is what my assistant did today to get it to work:
Opened notepad in administrative mode (%systemroot%)
Navigate to hosts file (%systemroot%\system32\drivers\etc\[hosts]) and open
Change the line of localhost on the left (mine was ::1) to 127.0.0.1
oh, you probably ran into this issue: http://www.zimbra.com/forums/general...osts-file.html