Hi Mendopaul,
It may be that there's been an accumulation of Word's temporary files, to the point they're interfering with opening the files. This is especially likely if Word has crashed or you've turned off the computer without exiting Word. See:
http://support.microsoft.com/kb/211632. For something more specific, see:
http://www.gmayor.com/what_to_do_when_word_crashes.htm. If you close all Office programs and navigate to the folders indicated in that article, you can safely delete any files with a .tmp extension or a filename beginning with ~. Do note that some .tmp files may still refuse deletion - that's OK.