By default, if you try to modify your hosts file in Vista, it will not let you save it.  It tells you that you don't have permission.  To successfully modify the hosts file, run notepad.exe as an administrator and open the file.

1)  Browse to Start -> All Programs -> Accessories
2)  Right click "Notepad" and select "Run as administrator"
3)  Click "Continue" on the UAC prompt
4)  Click File -> Open
5)  Browse to "C:\Windows\System32\Drivers\etc"
6)  Change the file filter drop down box from "Text Documents (*.txt)" to "All Files (*.*)"
7)  Select "hosts" and click "Open"
8)  Make the needed changes and close Notepad.  Save when prompted.