I've seen this bug since the very beginning of beta for the current release product. When I upgrade the parallels tools, the time synchronization services always gets lost and I need to uninstall and reinstall the tools again. I think it will be better in later release but no, I get the same result even though the release product. Fix it please. (I use Windows XP and, Windows 2003.)
What exactly is failing? Are your selections in the Parallels Tools time sync reverting back to default? Did you disable Windows default time sync to time.windows.com in the control panel of Windows?
An error log always says that it cannot find the file (I think executable) for Time Synchronization Service. This happens everytimes when I upgrade Parallels and reinstall the Parallels Tools. Why I know this? Because I have multiple harddisk images to maintain. It is very bored to uninstall the Parallels Tools and reinstall it again to get Time Synchronization Service working (not shown an error in log).
