Not sure about it letting cr*p sneak past it, but I do know that it has a bit of a reputation for hogging system resources.... and I don't think McAfee is much better either.
If you insist on getting a paid for Anti-Virus, ZONEALARM Internet Security Suite is supposed to be the best around at the moment (at least it came tops in WHICH? Magazine in February).... or if you want to save you money, Avira Anti-VIR PersonalEdition is quite good (what I'm currently using while I'm skint).
Nortons not very good,alot do get past it.Been using Mcaffe for the past couple of years,but found this tobe the same.Currently using Kaspersky,found tobe VERY reliable,dosen't let anything past it what so ever.
There's not a single program out there that will protect you 100%.
Norton pretty good apart from as jduck has already said.
My bro is a wizz kid at programming and he showed me how using flash/macromedia you can wipe a drive just viewing the page something to do with the source code and windows/back door entrance or something down that line, hell I get well confused but if I get a prob he soon sorts it for me.
He tends to use his network gear security I think (me pretty dumb with comps) and has several computers not linked to the net which he works from. I think one for web page work and programming and another which is heavly clocked for gaming with all that cooling stuff fitted.
Every day new nasty programs are released on the net and they can only be made safe once the software is known which is why no system is 100% safe.
I've been lucky so far but keep mainly to a few sites.
I've Norton on this old laptop and so far no probs if a little slow but bro's pc is as quick as can be with lots of apps running in the background.
Keep regular backups. I use Norton ghost which is very good or I should say bro does this for me