Hi Leo,
Please uninstall ooVoo once again and then re-install a fresh copy in a folder other than the default one (during the installation process choose a different location than C:\Program files\).
If that still doesn't work, please Exit ooVoo before trying to Sign in (Right click on the ooVoo icon located on your task bar and select Exit), and then cut the ooVoo.exe file from the new location you installed it and paste it to another folder (you can try the Desktop).
Please let us know the results.
Thanks,
ooVoo Dave