So how many people use installed anti-malware, registry cleaners, or similar? Do you think these are necessary?

I haven't had any AV(or 'cleaners') installed for over a decade. I use online scanners(mainly jotti) occasionally and that's it.

AFAIK I've only gotten one infection in that time, and I'm fairly certain it came from using FireFox(vulnerability, remote), so I uninstalled it. I was easily able to identify the malware, as it basically maxed CPU usage and also spiked network traffic(afaik it was a SPAM trojan). Unlocker assistant easily deleted the file. Problem solved. Only use Opera now.

What vectors are you most concerned about? Vulnerable/exploitable software, manual execution, protecting your system from other potentially naive users(children, spouse, friends), or perhaps something else?

My basic "detection routine": Open task manager, sort by descending CPU Time(another good one is page faults), and go down the list. If it's not using CPU then it's likely not doing much of anything, and likely isn't malware.

I suppose the closest app I use is Secunia PSI, which is a great way to get an overview of what apps need updating(or uninstalling) on the system.