Your explanation, re: the "uniqueness" of our different computers is clear - thanks.
I'll forward your recommendation for Flash Disinfector to my husband, in addition to my using it to check the external HD's and flash drives.
As you suggested, we'll check with our ISP regarding unauthorized outgoing e-mails and see what info they can / will be willing to provide.
I'll let you know how that goes, too - if you'd like?