When you trying to modify the "Windows\system32\drivers\etc\hosts" file to add some local dns mappings in Vista, the system will not allow you to save the file. It's because of the tighten security in Vista.
to solve this, refering to this KB, run your editor by right-clicking it and "run as Administrator", then will be able to save the file.
or by referring to Matthew's post, just grab the ownership of the HOSTS file so that one will be able to edit it without having administrator priviledges.
(by running cmd.exe as administrator...)
takeown /f c:\windows\system32\drivers\etc\hosts
icacls c:\windows\system32\drivers\etc\hosts /grant yourusername:f