I'm not familiar with ESET, but it sounds as if your colleague has network discovery enabled and/or file and printer sharing enabled. This is not necessarily dangerous, nor should it slow down your internet connection.
Port 139 is used by NetBIOS.
Are you the administrator for this system, or is this a school/work/loaned system?
I ask because modifying any settings related to NetBIOS can cause major issues with printing, shared network storage and active directory.