As VirusScan and Threatfire are both anti-virus applications you are going to have to choose between one or the other.
The rule of thumb is only 1 each of anti-virus, firewall, anti-phishing filter and pop-up blocker, as duplicates tend to fight with each other and actually reduce your protection.
You can have several anti-spyware applications installed however, as long as none of them have any kind of built-in protection turned on, such as the "Teatimer" in Spybot for instance.
We recommend the free ones listed in the first part of this thread:
http://community.mcafee.com/showthread.php?t=136913 Windows Defender is OK - I use it too.