Which are the compatible firewalls software with avast?
Comodo Firewall Pro. It’s free and i use without problems.
A third vote for Comodo.
Personal Firewall Tests & Results. Firewall rating:
http://www.matousec.com/projects/windows-personal-firewall-analysis/leak-tests-results.php#firewalls-ratings
Freeware firewalls:
http://www.firewallleaktester.com/tests_overview.php
http://www.thefreecountry.com/security/firewalls.shtml
Reviews:
XP: http://forum.avast.com/index.php?topic=27646.0
Vista: http://forum.avast.com/index.php?topic=27647.0
I have a lot of issues with the new version of COmodo 6.1 and avast 8.0 : boot time went from 30-45 seconds to 6 and 19 minutes (even with all avast shields off). They just don´t get along. Tried all different settings in both avast and Comodo , took a whole lot of time, but on none of my computers it improved anything. The only short-time solution is another firewall i think, so I would like to know an alternative to Comodo.
Depends upon your OS. If you are on XP then a 3rd party firewall is necessary. But if your on Vista, Win 7 or 8 there is no need for a 3rd party firewall. Especially if your behind a router with NAT. Its just paranoia. Windows Firewall w/Advanced Security can be configured to be just as effective if not more then Comodo and or any other 3rd party firewall. And its already on your machine. Why add extra processes.
Thank you for your answer. I have XP SP3 (home & pro), i know the built in firewall is not very good and not up to standards. I will try to use the old version of comodo (6.0) that worked, and prevent it from updating. It would, however, be nice to get to know a (free) firewall that is compatible with Avast 8 and could replace it, in case troubles of updated versions of Comodo persist.
Here are links for free firewalls with HIPS.
http://www.techsupportalert.com/content/hips-explained.htm
http://www.techsupportalert.com/best-free-firewall.htm#Quick_Selection_Guide
http://www.filehippo.com/download_outpost_firewall/
http://www.softpedia.com/get/Security/Firewall/Online-Armor-Free.shtml
http://www.softpedia.com/get/Security/Firewall/Privatefirewall.shtml
http://www.softpedia.com/get/Security/Firewall/Comodo-Personal-Firewall.shtml
Instructions to install Comodo Firewall http://www.knightwithapc.com/2011/11/download-comodo-firewall/ 
Thank you, Para-noid, for your comment, I will try Online Armor first.
As said: Comodo fw 6.1 seems not compatible with Avast 8 and XP pro, it kept biting even if avast 8 was uninstalled (after Comodo 6.1 has updated); still long boot time. I had to manually clean up the avast registry-keys and remove comodo 6.1 and clean up that installation. It behaved like a malware infection, this Comodo update.
The update also seems unavoidable: COmodo 6.0 has updated itself whilst auto-update were off on 2 computers; so I will have to get another firewall.
By now three computers have been infested by version 6.1 of Comodo, boot-up time on all of them have gone through the roof at 6,10 and 19 minutes (before: 30 seconds to 2 minutes), it is no workable situation.
I like Avast, have been using it for years, so the Comodo fw has to go.
Thank again for all repies.
Glad to help. 8)
If you check out all the various signatures you will find a wide variety of set-ups.
Keep in mind that by the years end Microsoft will no longer be supporting XP.
Thanks, Aventador, for reminding me. A 3rd party firewall with XP will be a temporary solution, but a neccesary one.
By the way: I have bought licenses for windows 7 and 8 Pro, but not the (new) pc´s to install it on. I understand from comments on this forum that the internal firewall of 7 and 8 is pretty good, no need a 3rd party firewall, so the problem will evaporate.
And if your behind a router with NAT all the better.