Hi, your last question first - don't even attempt to find out where they came from, this would probably only lead you to more problems; they come from all over the internet.
I am not familiar with Norton, I personally don't like anything to do with NIS. However, I do believe it should be safe to delete any nasties, with fingers crossed that they don't come back again, as they have a nasty habit of so doing - at least, according to what I have read within this Forum.
If I am wrong about saying Delete, I am sure someone else here will soon correct me.
Finally - Never Never Never open any emails you do not recognise or expect to receive. Delete them first.
Cheers and good luck.