According to the Wireless/Networking page a hosts file is a list of computer names and their associated IP addresses. Hosts files are used by Microsoft Windows and other network operating systems as an optional means to redirect TCP/IP traffic in special circumstances. These files are not required to use ordinary network and Internet applications.

This file typically sits on your Windows PC without much happening to it.  However, there are times when a malicious program may make changes to this file to redirect your attempts to browse to a valid URL and send you to their spam or malware laden website.

In order to get everything working correctly one of the things you will need to do is get the HOSTS file reset to its default state so these unauthorized re-directions do not occur.  You can always manually edit the file of course but Microsoft Support has made it easier by offering an automated Fix it solution that will make this process much easier.

If you head over to the How can I reset the Hosts file back to the default? support article you will see a very simple and straightforward explanation on how to use the Fix it solution to fix your HOSTS file.

Plus if all else fails they give you the specific format for the HOSTS files for Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008 and Windows 7 which you can simply copy and paste into your HOSTS file.

By the way, if this HOSTS file change was due to a malicious piece of software then make sure you get that cleaned up as well or else this change will only be temporary.