How to reset the Hosts file back to the default?

To reset the Hosts file back to the default, follow these steps:

1. Click Start, click Run, type %systemroot% \system32\drivers\etc, and then click OK.
   
   Note If you are using a 64-bit version of Windows, type %systemroot% \SysWOW64\drivers\etc.
2. Rename the Hosts file to Hosts.old.
3. Create a new default hosts file. To do this, follow these steps:
1. Right-click an open space in the %WinDir%\System32\Drivers\Etc folder, point to New, click Text Document, type hosts, and then press Enter.
2. Click Yes to confirm that the file name extension will not be .txt.
3. Open the new Hosts file in a text editor. For example, open the file in Notepad.
4. Copy the following text to the file:
   
   For Windows XP or for Windows Server 2003

   # 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

   For Windows Vista or for Windows Server 2008

   # 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

   For 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

5. Save and then close the file.

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In order to make this whole thing simpler Microsoft came with a option called Fix it ,Which does these above job in 3 to 4 clicks .Go to  http://support.microsoft.com/kb/972034 

Click the Microsoft Fix it button and follow the install wizard  and your host file is now replaced with the Default one.