Well depends on how paranoid you are lol,
But joking aside only use one anti virus as two or more will conflict,
My setup is,
Antivir PE - antivirus
Spybot - spyware scanner
Spyware blaster - Stops nasties from websites
Zonealarm - firewall
Ad-aware - spyware scanner
Iv had this setup for many years and serves me well,
I run a virus check once a week,
I run spybot once a week,
I run ad-aware once a week,
And update them often usually when i see a notice in the forum i rush off and update,
My anti virus updates itself once a day automatically.
I hope you find this helpfull.
But as long as you got 1 antivirus 1 firewall and about 2 spyware scanners that should be enough.