Trojan.Zbot Activity 15

I have multiple user profiles (family login accounts) on a home computer.  Only when a particular ONE of the user accounts does Norton warn me shortly after logging in with the following message:

"Norton blocked an attack by : System Infected: Trojan.zbot Activity 15"

Norton Internet Security virus definitions are up to date.

Ran full system scan (nothing found)

Ran Norton Power Eraser (nothing found).

Ran Norton's Poweliks Trojan scan/removal (nothing found).

Ran Norton's Necurser (nothing found).

I'm glad Norton keeps blocking the intrusion attempt, but I need to remove whatever is causing this to keep happening.  

Again, it only happens when one of the user accounts is logged into (and it unfortunately happens to be wife's so doesn't want me to delete it).

I'd rather not have to use other 3rd party software/downloads...I don't understand why Norton won't catch?  Is it too new of a strain?

Any expert help would be appreciated (and get me out of the dog house).