It is very simple to password protect a document in Microsoft Office. Providing you use a good password; at least 8 characthers, a mixture of upper case, lower case, numbers and symbols) your document will be relativley secure.
Please see an earlier blog post How Strong is you password? for more information.
Password Protecting a Microsoft Office 97/2002/2003 Document
Tools > Options > Security
Password Protecting a Microsoft Office 2007 Document
Save As > Tools > General Options
