Arquivo HOSTS - Editar e Redefinir para o padrão

Para editar o arquivo hosts temos que abrir o Bloco de Notas (notepad.exe) como Administrador:
No Windows 7 clique no botão Iniciar, Todos os Programas, Acessórios e clique com o botão direito do mouse no Bloco de Notas e Abrir como Administrador.
No Windows 8x digite na caixa de pesquisa notepad e logo abaixo clique em Abrir como Administrador.
No Windows 10 clique no botão Iniciar, Todos os Aplicativos, Acessórios do Windows e clique com o botão direito do mouse no Bloco de Notas, Mais e Executar como Administrador.

No Bloco de Notas clique no menu Arquivo e Abrir, navegue neste caminho:

C:\Windows\System32\drivers\etc

No canto inferior direito troque o Documentos de texto (*.txt) por Todos os arquivos (*.*)

Agora dê duplo clique no arquivo hosts, faça suas alterações e no final vá no menu Arquivo e Salvar.

Para redefinir, ou seja, voltar ao padrão original da instalação do Windows copie o conteúdo dos exemplos abaixo conforme sua versão do Windows, cole no Bloco de Notas e salve como "hosts", tem que ser entre aspas para não ficar com a extensão .txt. Copie este arquivo.

Acesse o caminho do arquivo hosts pelo Windows Explorer (C:\Windows\System32\drivers\etc), renomeie o arquivo a ser substituído para hosts.old e cole o novo arquivo hosts que você criou.

Arquivo hosts do Windows 10

# Copyright (c) 1993-2009 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
#      102.54.94.97     rhino.acme.com          # source server
#       38.25.63.10     x.acme.com              # x client host

# localhost name resolution is handled within DNS itself.
# 127.0.0.1       localhost
# ::1             localhost

Arquivo hosts do Windows 8x

# Copyright (c) 1993-2006 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
#      102.54.94.97     rhino.acme.com          # source server
#       38.25.63.10     x.acme.com              # x client host
# localhost name resolution is handle within DNS itself.
#       127.0.0.1       localhost
#       ::1             localhost

Arquivo hosts do Windows 7

# Copyright (c) 1993-2006 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
#      102.54.94.97     rhino.acme.com          # source server
#       38.25.63.10     x.acme.com              # x client host

# localhost name resolution is handle within DNS itself.
#       127.0.0.1       localhost
#       ::1             localhost

Arquivo hosts do Windows Vista

# Copyright (c) 1993-2006 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
#      102.54.94.97     rhino.acme.com          # source server
#       38.25.63.10     x.acme.com              # x client host

127.0.0.1       localhost
::1             localhost

Arquivo hosts do Windows XP

# Copyright (c) 1993-1999 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
#      102.54.94.97     rhino.acme.com          # source server
#       38.25.63.10     x.acme.com              # x client host

127.0.0.1       localhost

Veja também a postagem:
Arquivo HOSTS - Visão Geral

Fonte:
Microsoft