My computers only use LAN (not use Internet)
- Should I buy Avast Pro or Avast Internet Security?
- Should I install ZoneAlarm with Avast?
How many computers do you have?
If you have 3 or less, may be less expensive to buy AIS since one AIS license is good for 3 PCs vs. buying Pro for 3 PCs. If you’re already using avast! Home (free) check the update may be able to get a decent AIS upgrade discount.
FYI you can do an AIS custom install to only install avast! 5 AV without the AIS Firewall and Spam Filter–that’s what I did since I had recently renewed my Online Armor license for 3 PCs shortly after avast! 5 was released.
Cannot give any advice as to ZA. I did use ZA from when it was first released and kept using the last version of the 5.x ZA Pro branch (until switching to OA) since did not care for all the non-firewall features/bloat added to versions 6 and 7, and the amount of users have problems/issues with ZA 6.x and 7.x versions.
What do you use your computer for? Business, non-profitable activity? Home use?
my computers use at company (computer do not connect Internet, only use LAN, no business, no secret data.
I only want block worm (inflect from network)
at my company, virus attack by network (LAN)
NO! ZoneAlarm is not good and also often conflicts with avast!
ZoneAlarm Free & Pro are excellent firewalls.
ZA Free was the first software fw that I used.
No conflicts with avast!.
There was an issue with ZA Pro privacy settings (I think) and avast!,
but I never had a problem with it.
I give my highest recommendation to ZA Free!
A looong time ago i would have agreed with you, but not today, sorry. 
I have experienced different problems with ZA and avast on different systems.
Asyn,
I see in your sig. that you use Comodo fw and you say you had problems with ZA? :o
I feel 110% certain your troubles were with some other software(s).
You were wrong to give meigyoku a false impression of ZA Free.
Hm…recently people were having problem with avast and ZoneAlarm together
Shiw Liang,
Look in my sig., I’m using avast! 5 with ZA Free and I’m not having any problems.
Sometimes the wrong software(s) get blamed for conflicts & problems. 
I’ve tried all the free fw’s and some paid versions, I keep coming back to ZA Free.
ZA Free works great with 7 64bit, but I prefer a pure 64bit fw on my 7 64bit system
so I use Windows 7 Firewall Control.
One reason I like ZA Free is it’s trouble free. 
The only negative about ZA is the ZA support forum is the worst security forum on the Internet!
But that’s not much of a problem 'cause ZA Free is easiest to use & user friendly. 8)
Use whatever firewall you want to use! ;D
I too have been using ZoneAlarm free for a long time. No conflict with avast whatsoever.
I ran ‘Shields-Up’ and ‘Memory-Leak’ tests and all pass just fine. I also like ZA because it’s been trouble free.
YMMV however.