Arial Rounded MT
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This font is a development of the widely used and versatile Arial typeface. Rounding the ends of the strokes imparts a very different feel to the typeface, especially in the heavier weights, where the rounding is more apparent. You'll find Arial Rounded MT Bold more informal than its Arial counterpart.
These links will take you from the Microsoft web site to a Monotype Imaging web site. Monotype can provide many common Microsoft supplied fonts under license from Microsoft or under license from other font vendors.