A friend had a similar problem uninstalling CAV from a Mac Mini running OS 10.7.5Read all these instructions through, before actually using them, as there may be some difference in how you want to proceed.
In short, you have to use the Comodo "Uninstaller" that is bundled with the "Installer" download package. The "Uninstaller" may be available by double-clicking via the Finder>Applications>Comodo>Unistaller
Since the "Uninstaller" was not available for me, I had to download and re-install an old version of CAV (which was no longer on the Mac).
The latest version of CAV on the Comodo site would not install, as my friend's OS is too old.
I found an old version here: http://download.cnet.com/Comodo-Antivirus-for-Mac/3000-2239_4-76171344.html
I then installed the downloaded version onto the Mac - But be Careful
during the installation process to not install any Adware that is bundled with the download. I had to de-select several adware items.
The old version I used (after the Zip was unpacked) is called:Comodo-AV-2011-11-03-v.1.1.214829.106
I could have run the "Uninstaller" that was bundled in the package, but I decided to reinstall first so all file associations would be re-established. My guess was that the "Uninstaller" would work better that way.
So, after installing the old version of CAV (and restarting the computer - follow the on-screen instructions), I double-clicked to Finder>Applications>Comodo>Unistaller. I ran the Uninstaller, then restarted the computer as specified. After a few minutes, the computer restarted, and Comodo had been fully uninstalled. You can check "Applications" to see if it is listed.
I was then able to install "Avast Mac Security" which had been previously blocked by the remnants of Comodo on the computer.