You cannot modify the Hosts file or the Lmhosts file in Windows Vista
To work around this issue, follow these steps:
1. Click Start Start button , click All Programs, click Accessories, right-click Notepad, and then click Run as administrator.
User Account Control permission If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.
2. Open the Hosts file or the Lmhosts file, make the necessary changes, and then click Save on the Edit menu.