What I would do if you’ve still got the problem with that one specific computer is contact your ISP support department and check with a technician as to what might be causing your problem in regards to your network settings.
I say this because the problem is clearly not an Avast issue (since as you stated you had uninstalled Avast and I might add you’ve mentioned uninstalling Malwarebytes as well and the issue persisted unfortunately).
So because it apparently can’t be Avast and by flushing your DNS cache as you presumably have at this point and the fact that this issue occurs on 3 different browsers (Chrome, IE and FF) then it would seem to narrow down to a network issue that your ISP support should be able to hopefully resolve for you. The fact that one computer does work may possible help your ISP support as far as running a comparison check on what might be the difference between the two computers.
If you have any more thought or questions on this of course be sure and post them here and of course however the issue gets resolved let us know if you will, it could possible provide help for someone in the future.
