Backdoor.IRC.Flood.E & Backdoor.Dvldr

I have a user who has been "infected" with the above two Trojans. They have both been a real hassle to try to remove from her machine. I spent close to an hour a couple nights ago cleaning up the Backdoor.IRC.Flood.E Trojan, deleting any and all references to it in the file system and the registry, only to have it reappear again this morning.

The Backdoor.Dvldr Trojan evades even being seen on the machine in the file system or in the registry, yet Norton continues to detect it.

I have followed all instructions on the Symantec web site for cleaning these up, but to no avail.

My biggest question at this point is "where do these Trojans get in... what is the method of infection? And of course how can I eradicate these things without resorting to a complete format and rebuild?

Any and all help would be appreciated.