I would take the position that, if you have any sort of connection to the internet, then Antivirus, Antispyware and Firewall software are all of equal importance and usefulness - i.e. essential!
Imagining the increasingly unlikely scenario of a machine or network that never has - or ever will be - connected to the internet, then you'd still need a virus checker to prevent viruses being introduced on removable media.
I guess by that line of reasoning, it could be suggested that a virus checker is "more essential" (yes, I know) than the other two.....?
At present, I'm using the following free products at home - always open to suggestions and dire warnings, though:
Firewall: An old PC running a linux-based dedicated firewall. ZoneAlarm v3.1 served me well when I was connecting to the internet through a Windows PC.
Anti-Spyware:SpyBot Search And Destroy v1.4. I keep hearing good things about the new Microsoft one, in spite of my deep and long-held suspicions about Gates and All His Works. There was a very interesting roundup of the current crop of Anti-Spyware products in the November edition of Personal Computer World. Noticed that Ad-Aware didn't come out of it particularly well.
AntiVirus: AVG Free edition v7.1.
Whatever security product is used, though, it's vitally important to regularly check for and apply updates.
Well that's my 2p worth.
PB