It’s a built-in safety feature. There is an easy workaround.
- Click Start -> All Programs -> Accessories.
- Right-click Notepad, and then click Run as administrator. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow or Yes.
- Open the Hosts file (from the notepad you just opened, at folder: C:\windows\system32\drivers\etc\) , make your changes, and then click File -> Save.