Is this a fine working combo? Is there any known issues?
OS will be Windows 7 64-bit.
Care should be taken as the comodo firewall isn’t stand alone but also comes with an AV and that would conflict with avast, so when you install CIS, you need to do a custom install and deselect the AV element or subsequently uninstall the comodo AV element.
I think they have a separate download for firewall only now, but step thru it carefully to make sure. Make sure you get the right version. There is 32 bit and 64 bit. I don’t have any problem with it. I use the firewall only.
Joe
I’ve been using an older version of CPF v.2.4.19.185 and Avast AV all updates, for a couple of years now with NO PROBLEMS.
One possible issue could be that both Avast AV and CPF have “Behavior modules” possibly resulting in a slow down of processing. ;D
Still a good fit 8)
I’ve tested Comodo Firewall 5 beta with Defense+ side by side with avast 5.
No problems. You’ll have the sandbox, the HIPS (of Defense+) and also the cloud scanning (cloud antivirus) even without installing the CAV.
Yep, it works just fine. I have tested this combo in a VM and it was good, very good. Make sure, as others have said, to install without the AV when presented with the choice.
Thanks for the answers 
I will have a ADSL-modem. It has a FW. But i think it can be a bit tricky, when need to open ports etc. Or should i use it? Or go with Comodo? Or both?
You can use both. Hardware firewalls does not supply outbound protection. Software firewalls do.
What about Port Forwarding with the modem? Do i need that for every app which acces the net?
I really never had patience to do that…
I did it once when I want High ID at P2P programs (and not Low ID when you do not forward ports in your router). This require a stable internal IP and I can’t do that as the IP is given by DHCP for me.
So better to disable hardware FW?
Let it on… just do not worry about port forwarding…
Does it slow things down not to port forward?
No. The hardware firewall will only block incoming (hackers) connections I think.
Do you use P2P? No, so don’t worry about port forwarding.
Well, it’s my opinion due to my lack of knowledge. Hope that someone more expert could say if I’m wrong.