I have written this tutorial according to Microsoft Office 2003, although the same principle will apply for all versions.
For Office 2003, in the open file click on the File button and select Save As, this will open the appropriate dialogue box.
Now click on the Tools button and select General Options if it is an Excel spreadsheet or Security option if it is a Word document.
You will now be presented with a box in which you are offered Password to open and Password to modify. You may either password option, or even use a separate password for each option to enable certain people to read the file but not change it.
For Office 2010, click on the Office button and select Prepare then Encrypt Document to restrict access or select Save As then Tools and General Options to restrict modifying the file.
