Hi again,
I have never used yahoo groups, so don't know anything about them. I just did some searches in their questions and answers sections and didn't really find anything too helpful that's for sure.

You said it was guigrp.yahoofs, but wondered if it's actually gui.grp.yahoofs ... with another dot in between the gui and the grp?

gui is a graphical user interface [graphical user interface, often abbreviated as GUI, is a type of user interface using graphics for interaction with a computer or other media formats which employs graphical images, widgets, along with text to represent the information and actions available to a user. The actions are usually performed through direct manipulation of the graphical elements. - got that def. from Wiki]

grp is obviously groups.

and yahoofs... well that's a mystery to me but I am guessing that yahoo uses that as one of their groups servers.

However, if you've got pop-ups blocked correctly you shouldn't still be getting that pop-up. I would imagine you've double checked that since it came back from the shop.

There is no website called yahoofs.com
I couldn't find anything in ARIN Whois for some reason but I found something in CoolWhois, whatever that is.
http://www.coolwhois.com/d/yahoofs.com

I'm only finding the same sort of stuff that curious is finding... lots of talk about spyware, trojans and not much else.

I'm sorry I can't help you more with it, but keep hitting dead ends.