I don't know any complete free library, but to improve the look or functionality of ordinary .Net elements, I would look at the CodeProject site. There are many articles on customizing controls. For example, here is about creating round buttons.
You may need to draw a drawing of the owner, but assuming that the license for the code in this or that article is suitable for your needs, you may be able to reuse the code, do not write a lot, and some of the articles contain which you could simply indicate in your code.
If you want to provide your Windows 7 application, also see the Windows API Code Code for .Net
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