Many AV programs, you can set the depth and scan options ...depending on the configuration, some may appear slower than others due to the depth of scan...the deeper something scans the slower it is, so there's a balance you have to consider.
AVG free is nice, the virus definitions are the same as the paid versions, you just get less bells and whistles which typically are not needed anyways....they tend to be bloat to me.
Also, as I type this, people are creating new virus/malware. All AV software companies are always a step behind. So even though you may feel you may have the safest AV, if someone created a new virus, and obviously won't be part of any definitions update, all will likely miss it during any type of scan.
Aside from AV software, try to avoid using JavaScript and other scripts that are used for most sites. I use a program called "no script"
NoScript - JavaScript/Java/Flash blocker for a safer Firefox experience! - what is it? - InformAction which is a fantastic way to prevent all scripts from running when you enter a site. You have to have a bit of knowledge of how to use it, but it gives you the option to allow/disallow scripts from running on a site. You'd be surprised as what runs when you enter a domain. This is a very good way to prevent malicious software from unknowingly running on your computer.
I can't tell you the last time I got a virus. I use a sandbox for testing software which I'm unsure about...
Good luck!