Maybe you tried to uninstall it with insufficient privileges so of course you won't be able to. If you have, then the setup files might be corrupted.

Try to re-install it again and after that, try uninstalling it now and hopefully it would work.

Deleting the folders and files manually will leave some values in the registry and would put you in a bigger trouble as you have to delete those from the registry one by one and your operating system may not work at all if you delete an important file accidentally.

Make a backup of the registry before you try modifying anything.