No, you can have 2 if you want. Two of anything really, it just means that you have to see which 2 work the best together without conflicting. Trust me, I've done just that for the last few months! Avast Home doesn't conflict with Threatfire, but it DOES conflict with Comodo's "Firewall". But not with Comodo's "Memory Firewall", which a different thing alltogether.

I've gotten avast home, comodo "memory" firewall, pctool's threatfire, AND Windows' own programs to happily coexist without skipping a beat. It reports that having 2 firewalls can cause conflicts, so sure, if it causes conflicts on your particular system (nobody can tell you really) then adjust accordingly. I've also gotten them to work really well in Windows Vista, though it's on my sacred laptop and (if I could) I'd rather pay the premium for either Kaspersky or Norton.

edit:

In order to yeild a little here, yes it's absolutely preferable to have only one program for each purpose. and that decision is easier to make when you buy your AV program and it's as fully featured as the aforementioned. But when you're depending on FREEWARE, it doesn't hurt to experiment and find a couple that can cover the bases. You already know that one's going to have a couple features that the other doesn't have.

Yes I know
I've done this too, back when I had Sygate and perfectly configured to allow the other Firewall I had at the time

After many years since then, I've now realised that there can only be one (and use the force too )

Just use 1 Firewall to do the lot.

Observe the reports. Go into your event reports and see what's going on. If the logs are cluttered with inconsistencies (ie, failed services or resource usage warnings) then uninstall your AV or firewall and see what happens. If it's the firewall, set it to lesser aggressive settings.

Yeah, you're right. It would seem kinda paranoid anyway, huh! Also, where it can be applied The Force is always welcome

I used to swear by norton corporate - I wouldn't use norton home products, but I use aging systems and norton started really bogging down my systems. Now I use clamwin and it's great and not system hogging at all. Best of all it's open source and free.

i use norton, hahahah

crap i didn't know,

AVG & kaspersky, I prefer one of this they are best.