Utterly confused...

Hi all... for the third time in the past 3 weeks my wife's email contact list has been partially hijacked and various stupid emails have been sent to about half her contacts.
As a simple and expedient measure this time, I had her go to a computer we reserve for use only by our grandchildren and change her password. This computer has parental controls set and it is only used for disney and parentally approved kid's sites.
But I'll be damned that in less than 6 hours, another batch of :crap went out to a selected group of people off her contact list.
FYI... I even had her change her security question for her password to something which has no connection to us nor frankly makes any sense whatsover.
As you all know, on a scale of 1 to 10 for computer savy, I am a 1.
But even I can figure it can only be be one of two possibilities; either her 'email account' itself is contaminated and when she opens it she reinfests the computer she is on, or our router is infested. These are the only two things in common. FTR... she is only reading the occasional email from freinds on her contact list and as a precaution she never opens any attachments.
Is there some way I can look backwards at the router and scan it? We think it does have a firewall, but the paperwork is long gone and I cannot figure out how to set the firewall up. I think it requires the original installation cd to change the settings.
I can certainly run the Malware section on both puters, but frankly team, I'm about worn out. We have 3 laptops and 3 desktops and collectively we have had 5 to 7 what I would call major infestations within the last 3 months or so.
And I don't mean the typical run of the mill adware either.
Let me re-emphasize, the computer she used to change her password and check her email is reserved for disney and the grandkids and the only other internet action outside of the parental settings is my wife going onto hotmail to change her password.
:banghead :guns :tas :heli :*** uke
I think that says it all..
:major

 

:-o Sorry... a little more info..
The disney computer has MSE and malewarebytes installed, and my wifes laptop has avast and malwarebytes. And oh yes, the standard MS firewall set to no exceptions on both.
All the free versions, but never the less.
:wave