You can assign a password to Office Documents, which will allow only those who know the password to open it i.e., without the password the document cannot be opened. Look here:
http://netsecurity.about.com/od/freq..._encryptms.htm
As Snowmonkey mentioned, no password is foolproof and if someone really wants to break it they can, so choose an obscure password (a random sequence of letters and numbers) if your docs are that important to you.