PopUp Network is an adware hijacker.  It will take over your computer by way of hijacking your browser and redirecting it to a site of its own choosing.  It will also tamper with your browser settings.  The homepage you took the time to set is no longer available, and you are now on a site that you did not ask for or give permission to change.
PopUpNetwork was probably installed onto your computer when bundled with a free software package.  It is junk software and should be removed immediately.  Choose an anti-spyware program such as SpyZooka to get rid of this problem.  Your computer will be up and running like new after SpyZooka is done with it.