For quite some time, my computer at work has exhibited many signs of a malware infection, but none was ever to be found.
More specifically, at random times (usually when quickly typing a URL into a web browser) I would end up on the wrong page - usually a page trying to convince me that I'm wanted by the FBI, or trying to sell black market Viagra ('scuze me, "V1@gkr4"), or other kinds of annoying spam.
Of course, when you see something like this, you immediately assume you are infected by a virus or have some kind of malware. And I did all the right things - boot from a known-good recovery disk and run all kinds of scanners. And none of them revealed anything.
So what was going on here? The answer is that I got hit by a perfect storm of coincidence.