How can we give password to any ms office file...

ya ms office had a inbuild password security feture.

to access it go to tools menu->options.

in the options menu go to security tab.

there u can see password to open and password to modify.

password to open prevent unauthrosied persons to open it where as password to modify will make open the document but prevents changing/modifing the content of it.
(the above is for office 2003)

in case of office 2007 click on the office button (fount in top left corner)

and prepare->there u can select encrypt document or restrict permission depending ur needs.