Avast free edition.........anti-virus/malware only, no firewall.......will and does run very happily alongside Malwarebytes free edition, which is of course an on-demand scanner.
I have these two and the real time protection from Avast coupled with a manual scan now and then by Malwarebytes is a very good combination.
The built in Vista firewall with it's two way protection will work fine alongside Avast.....and is robust enough unless you are a visitor to dodgy sites.
Some free firewalls (Comodo/ZoneAlarm/OnlineArmor as examples) have been known to conflict with Avast.......I use the free Outpost firewall which is light, easy to configure, reasonably secure and lives happily with Avast.