If you attempt to install Symantec Antivirus Win64 from Win 7 without the uninstall password, you'll probably find that people suggest that you change a registry key value:
HKEY_LOCAL_MACHINE\SOFTWARE\Intel\LANDesk\VirusProtect6\CurrentVersion\AdministratorOnly\Security\UseVPUninstallPassword from 1 to 0.
That doesn't really work for the 64 bit variant as the key locations are different. I looked around a bit and found that the key was here, instead:
After changing that 1 to 0, you should be able to uninstall without a password.