I currently run Avast Pro AV (no firewall) and ZA free firewall (no AV).

I just got an attractive offer to upgrade to ZA 2012 AV+Firewall but before I plop my money down I'd like to know if having both ZA AV and ZA Pro (with AV) will cause problems. Stuff I read on the net says it's a bad idea to have 2 AV programs running.