(Wow, this was a really old topic you posted in)
Anyways, I have found that disabling AVG's Identity Protection helps for me (my Comodo installation hasn't had any problems for months now ever since I did that).
To allow Task Manager to terminate Comodo, open the UI (if you can), and go to Defense+ -> Computer Security Policy (you may have to click "Yes" if there's a prompt).
Scroll down to the "Comodo Internet Security" group, click "Edit", then "Customize", then go to the "Protection Settings" tab.
Have "Active" checked for "Interprocess Memory Accesses" and "Processes' Termination" (if it isn't already checked), then for each of them, click on "Modify", then add the Task Manager process.
That will allow Task Manager to kill Comodo, but it won't help with the shutdown problem, unless you make sure Comodo is responsive (i.e. the UI will open) every time before you shut down, and kill cmdagent.exe if it isn't working.