http://office.microsoft.com/en-us/tou.aspx
See the EULA, pay close attention to section 1.2
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)