How do I know if my Mac has a Trojan?

Hi,

I have a MacBook Pro that is less than a year old. I have recently become very worried that my computer has some sort of Malaware or Trojan (these are terms I picked up from Google-ing the issue).

Recently I have been running into problems logging into accounts that I routinely access. I cleared my cookies in Chrome and all of a sudden there was a random email address in the username portion of my Facebook account. I am worried that I accidentally downloaded something harmful that is trying to get access to my private accounts. I admit that I stream videos online and do get pop-ups that I always shut down, but it's possible that I accidentally clicked on "okay" instead of "cancel". I ran an Activity Monitor and I don't see anything out of the ordinary, but it is hard to tell what is normal.

I also went through the steps listed in this article:http://www.forbes.com/sites/adriankingsleyhughes/2012/04/06/mac-flashback-trojan -are-you-infected-how-do-you-remove-it/

The test came back clean but is this a trustworthy process?

Is there something I can do to completely rule out that I have a virus or Trojan? Or, am I just being ridiculously paranoid. Thanks in advance for your help!