Basically you'll need to edit the file as administrator. The easiest way to do this is to launch notepad as an administrator. In the Start menu -> search box type "notepad"
Then right-click notepad.exe and select "run as administrator." You should now be able to save changes to the Windows hosts file.
You can also create a shortcut to the Windows hosts file that will edit it as an administrator automatically (ie. associate the administrator permission with it) as follows:
Create a shortcut (on your desktop) to: C:\Windows\System32\notepad.exe C:\Windows\System32\drivers\etc\hosts
Then, right-click the shortcut, select "properties" -> shortcut tab -> "advanced" button -> check "run as administrator"