How to modify the Host File In Windows 7 Or Vista?

How to modify the Host File In Windows 7 Or Vista? I receive the following error in my machine Windows 7;

Save As
C:Windows System32driversetchOSTS.txt
You don’t have permission to save in this location.
Contact the administrator to obtain permission.
Would you like to save in the My Documents folder instead?
What is this error message? Please guide to properly secure.
