You have way too much security in the first place. Comodo has D+ which a HIPS. Avast Free 6 has a behavior blocker. Both together is not a good mix. The Windows 7 machine needs only Windows 7 firewall on. Any reason your so paranoid?
Well, the thing is that I do not have any trouble in my windows 7 ultimate edition computer using Comodo… That’s why I ask if anybody knows about any compatibility issues between Comodo/Avast and a XP SP3 Pc.
I’m not a Comodo user, but there are plenty of people here who do use Comodo FW with Avast. You do not necessarily need to use Windows 7 FW, although it is better than previous versions of Win FW’s. If you continue to have problems, you may want to upgrade to Avast v. 6.0.1091.
Anyway - I also think you are performing a security-overkill on your PC.
Win Fw + Avast Free + MBAM on demand + (I see you are using) Sandboxie = absolutely sufficient.