You can make a file read-only by navigating to it in Windows Explorer, RIGHT clicking on its name then selecting 'Properties' from the pop-up menu. At the bottom of the general tab in properties you will see 'Attributes'. If you then tick the 'Read-only' box you set the file read only. This will only protect the file against accidental changes since it is just as easy to un-set the attribute.
In 2007 and 2010 you can password protect the file against unauthorised readers but it adds no more protection against authorised readers !
There may or may not be any third party applications which will do what you want, it's not an area I know anything about.