Infostealer.Branco

Yesterday afternoon (9/20) I glanced over at my monitor and saw web pages flying on the screen, one after another at a very fast pace.  I thought that must be virus activity so I logged off my computer with a restart.  When the desktop returned, there was a note from Norton that I had a virus called Infostealer.Branco.  It further indicated that the virus had been quarantined.  Since this particular virus steals passwords, I am concerned about whether or not it had a chance to steal some of my passwords before Norton detected it.  In looking at the Norton Virus definitions, it appears that one became available for this virus on the same day (9/20); could it have been on my computer without me knowing about it and then was discovered when Norton updated the definition files.  In looking at the information that Norton gave me, it appeared to have originated from an email.  I had not opened any attachments that day.  It worries me that it may have been sitting there stealing passwords.  Is there any way to know?  I have Norton Internet Security on my computer and it updates my virus definitions frequently.  I also have the firewall active.  I would also like to know how I can be sure that it is no longer on my computer.