Random IE Pop-ups - Trace To The Root?

I know it is stupid but i have put up all the defense i can, firewall, antivirus, but somehow these nasty buggers just slip through. I am no computer expert, just an average user, so i don't want to reinstall the entire windows XP and go through all the update process.So the question is: how can i track down whatever is causing this random, time-to-time IE pop ups? When i do my research the term active scripting comes up all the time (for pop ups). I wonder if this is the culprit and will this thing remains active even I don't have IE windows opened.