I have not read this whole thread, so forgive me if I am repeating things others have said, or am just simply talking more than I listen.
Security software eating system resources is a pet peeve of mine. The bitter irony is that most of the time it does NO GOOD anyway, else all these people wouldn't be getting infested with malware. Think about it -- has the malware problem been 'abated' in any way by all these security solutions? Nope.
The #1 mistake novices make is installing more than one security suite. That is a huge no-no. It does NOT make you doubly protected. It makes you doubly slow, and doubly prone to potential strange problems.
My recommendation to all users, novices and advanced, is to adopt Microsoft Security Essentials. Since I am the author of an EXE compressor that is sadly abused by malware authors at times (despite my best efforts), I keep in touch with the security companies to help them combat this problem by 'scanning inside' compressed EXEs. I have really liked what I've heard from the PM for Security Essentials at Microsoft. They are doing it right -- trying to avoid the *very problematic* issue of false positives, while keeping people protected and using *minimal* system resources. And you know if anyone can make sure things are done as efficiently as possible on Windows, it will be Microsoft.
Security Essentials is 100% free and has just a few options. The options it has are the *critical ones* though. You can disable real-time scanning (the biggest impact on system performance), or tune it down to a number of different levels. You can exclude specified paths or file types. Perfect. They know this is needed to keep systems running optimally. You need to tell it to scan the risky stuff (such as incoming downloads and attachments, or removable media) .. and leave the rest of the system alone. After all, while it is theoretically more secure to keep rescanning every darn file that is opened, it is a bit absurd. Tune it down to only scan the incoming files, and be careful -- and you're gonna be ok in most cases.
I do not want to 'pick favorites' since I also deal with other companies, so I must also mention that if you want more ADVANCED controls and need even more enhanced security, the other companies -- you know the names (list removed as I feared I'd leave somebody out) --are getting better and constantly improving their software. They are also aware that they need to 'speed things up' and have offered similar options to help users do that through more selective real-time scanning.
So, that's my recommendation on the security products part of this discussion... which seems wholly OT, but ....