Microsoft Office icons are copyrighted by Microsoft and this prevents us from using them in non-Microsoft applications due to their licence terms.
I\'m trying to find so
Fatcow hosting has some OK ones, amongst other fantastic icons. You should include a link to their page in your credits. Not sure what "your credits" has to be but I guess the whole thing is a social & SEO ploy, if your app is on the public internet then you'll need to put it somewhere that people and search engines can find it.
For a private business app not on the public internet I guess you could bury the link fairly far down with a good conscience.