Well, the random pop-ups could be spyware or something like that. Get Ad-Aware from www.lavasoftusa.com ( I am not completely sure about the URL, but google it if that one doesnt work). And run a in-depth scan. Also, does the computer in question have anything that would cause extra ports to be open? (KaZaA, AIM, etc?) It may have come from there as well.

Hope that helps.