Here is what I use, daily:
Avast!4Home (by Alwil) - antivirus; web (activity) scanner; webmail scanner; mail client (Outlook) scanner as well an instant messanger & P2P (file share) client scanner.
This program will also allow you to forbid file writes (edits) & deletions, if you so do desire.

SpyBot Search & Destroy
Adaware (by Lavasoft)
^ I run both during my lunch break, or one first thing in the morning and one prior to my leaving for the day.

Weekly:
TrendMicros free online webscanner


Norton's suite of software, such as what they call Security, SUCKS ASS!
It is THE crappiest program that I've stumbled across, & I've stumbled plenty. I sold the junk for many years and once Peter left things went down hill.

Also, I stay away from InterNutExploded (IE).


As for what to rely on ... not-a-thing! They all work and they all let you down, sooner or later!

Them little pricks (double entendre) have nothing better to do with their time once they've painted their peckers orange. Bunch of good for nuttins'!