The folowing is from
http://www.microsoft.com/permission/copyrgt/cop-img.htm#CGL
Important note with respect to use of clip art: Certain clip art contained
in Microsoft products should not be commercially redistributed nor used for
commercial purposes. In addition, there may be other legal concepts you may
need to consider when using copyrighted material (including, but not limited
to publicity rights, trademark and logo rights). Microsoft will not assist
you in making a determination between commercial or non-commercial use, nor
can we provide you with legal advice as to publicity or trademark rights. If
you are having difficulty deciding whether your intended use is appropriate,
or whether you require written permission, or whether other legal issues
should be considered, we strongly encourage you to seek competent legal
counsel. If you or your attorney determines you are required by law to
obtain copyright permission, you may request permission for reproduction,
redistribution, or modification of the clip art through the appropriate
owner of the subject materials (as may be cited in the Microsoft product).
If, on the other hand, you or your attorney determines it is permissible to
proceed and include clip art from a Microsoft product, we do ask that you
correctly designate our trademark(s) when referring to our product(s) in the
notice or copyright portion of your paper, project, or product. (See
http://www.microsoft.com/trademarks/ for more information on appropriate use
of Microsoft trademarks.)
Even though your group is a non-profit organization, the use of the clip art
"could" be construed as commercial use. I doubt if your group wants to risk
the wrath of Microsoft. Let's face it, it's a sure-fire loss - and a
definite pain in the butt!
I'd recommend a contest for a new logo or contact a local art school and see
if someone wants to volunteer their skills.