Is it ever OK to use Microsoft clipart for commercial purposes?

Leaving aside aesthetic issues, the answer is ... not really. Microsoft and other companies have immense legal muscle and will often act to protect their intellectual property. In general, Microsoft has deemed that non-commercial use of their clipart is permissible, provided the clipart comes from a legal copy of a Microsoft program or was downloaded from Microsoft’s online clipart site (office.microsoft.com/clipart) by a Microsoft user. Clipart may not be on-sold.

According to Mr Gates' lawyers:

The following guidelines apply to the use of clip art:
1. You may use clip art in your school assignments and projects.
2. You may use clip art in your church brochure.
3. You may use clip art for personal, noncommercial uses.
4. You may not use clip art to advertise your business.
5. You may not use clip art to create a company logo.
6. You may not use clip art to illustrate the chapters of a book.

Given the sheer amount of hideous Microsoft stick figures in business brochures and the like, one might think that particular horse has well and truly bolted.

Fortunately, better clipart is available in many places on the web, some free and others rather less so. There is even a website devoted to storing the vector logos of the world's major brands.