If it are the same websites at both computers using the same browser that you can visit and can't visit, it's either the browser (try another one, Firefox and Chrome are free) or the common hardware (router, modem, ISP) or the same malware infection on both.
If one of them happens to be a laptop, try elsewhere.
I have two computers...a Lenovo and an HP Compaq Presario (Windows Vista Operating System). I hardly ever use it but today I was trying to look up some things and...to my surprise,,,for numerous (not all...just a lot) of websites, I kept getting "webpage cannot be displayed." I do not understand why I would get this message sort of random. What might cause such a situation and how might I be able to fix it?