Norton from comcast detected Trojan.Gen.NPE in opera browser cache

HI all so today norton detected Trojan.Gen.NPE  and one other inside opera browser cache i can't remember the name off the other one it said it deleted them but threw Trojan.Gen.NPE in quarantine i since deleted it, but not sure where it came from unless it was just from general search engine use, i don't go to shoddy website or anything like the sort.  is it safe to say since it was in a cache file it really was not installed, i since have done scans with hitmanpro and dr web cure it , along with a superantispyware scan nothing shows up now, so it seems gone. i even ran norton's own clean up tool it had for it on the info page for that one it did not find it installed. so was it indeed not really installed and just a esence of a trojan ??