No matter what you install or are running, you have to realize it is physically impossible for any AV to catch everything all the time. Not only do viruses change their stripes with regularity, but there are new ones always being released. AV's can watch for certain behaviors, but this doesn't always guarantee they'll catch something if it's new.
A new virus is released into the wild on day 1. The virus must first be recognized for what it is and samples submitted to the AV companies for analysis. After analysis, they must write a definition to combat it. This can take anywhere from several hours to a couple days depending on the complexity of it. During this time, the new virus is spreading as fast as it can.
When the solution is found, it must be tested, streamlined to run on various systems and then deployed. This will also take time.
The best defense you have is to run what you feel comfortable with, keep your AV Defs up to date as well as any patches for software, and practice vigilance.
You know how your PC normally runs. Does it look like it's doing something/running a little bit out of the ordinary? Then it's time to investigate. Have a look at your running processes (Process Explorer
) or see if you can still access your RegEdit.
But just because all "appears well" doesn't always mean it is. Have a look at the workings once in a while and see if anything new has popped up, and if it has, why.
Use sense when surfing. You can have the best rated AV, but going to shady sites or trying to D/L cracks will bring on something that will eventually overwhelm even the best AV.
Remaining vigilant combined with common sense is your best defense for staying virus free, but it demands some of your time and attention to do so.