http://office.microsoft.com/en-us/tou.aspx
See the EULA

If you have a valid license to Office, you can use the clips:

  Personal items
  Business items, under the following restrictions:

   1. The clip should not be used as a part of your logo.
   2. If you are selling an item which depicts the clip, the clip cannot be
the primary value of the item ( you cannot sell a clipart collection or sell
a book of just clipart, etc)