Quote Originally Posted by nightflier
The sad reality is that no virus scanner is going to catch everything hidden deep in the bowels of your computer. I have had cases where 3-4 different scanners still did not get everything. ...
Sadly true. Notably, there is type of virus called a "rootkit", (which see here), that is designed not to be detected. Typically there are used to allow an outsider to control your computer. Also, there was a least one notorious use of a rootkit by Sony (as I recall) to enforce DRM; they caught so much flak for that that they had to provide a removal tool. Rootkits are not typically caught by routine virus scans and sometimes can only be removed by ad hoc software designed to remove the particular rootkit.

Quote Originally Posted by nightflier
... Everyone has to find a balance that works for their own needs. You can spend a lot of time obsessing over security, but that's time taken away from work (and/or entertainment) - life is short, as they say. ...
Exactly