I've also come across several virus that have actually deleted Mcafee and Norton (and AVG ). The virus would actively unload most virus scanners while reloading itself to defeat most efforts to manually remove it. If you get one of those into your PC as a zero day infection (before your scanner is updated to recognise it) you are in deep doo-doo no matter that scanner you are using. Lots of fun using Hijack-this and Regedit while running windows in 'safe mode'
. Sometimes it's just easier to nuke the PC and reload windows.
Value for money AVG is way ahead, and I believe it works just as well as the expensive ones.
None of them are 100% secure.
Ian