Hi,
In fact I just tested with a clean registry branch, and I have the same error.
So Thanks for reporting this problem, will be fixed in the next release.
Meanwhile, as a workaround, launch Desktask, wait for the error, and then simply close DeskTask normally, either selecting Quit in the tray menu or right click the blank window and select Quit. Next time you launch DeskTask the error should be gone since the value for "WindowTop" is already in the registry.
Thanks
Regards,
André Santos Carthago Software Support
http://www.carthagosoft.net