It's your choice. Avast works well with a lot of the AS and firewall applications.
Do you want a resident antispyware, or demand scanner?
2 way firewall, or prefer to use the Windows firewall?
A fairly common security lineup includes 1 (and only 1) antivirus, 1 (and only 1) software firewall, and 2 or 3 demand antispyware scanners. AS scanners that have a good reputation include Superantispyware, MalwareBytesAntimalware, a2 (a squared), Spybot S&D, these are free, or have a free version. If you want to pay there's a few more choices.
Two way firewalls that seem popular include Comodo, Online Armour, ZoneAlarm, Kerio, or PCTools