Unable to find cavshell.dll. Hanging during update

I have Comodo Firewall Free 4.1.149672.916 installed.

When I tried to update, last week I got the error “Unable to find cavshell.dll”.

Since yesterday, when I try to update, after the downloading is over, the update hangs with the message

“Copying cavshell.dll file”.

Also, can I install the a version of CIS over the old one, or do I need to uninstall the older version first?