img = Import["http://i.stack.imgur.com/TlMUh.png"];
Since you ask this kind of basic image processing question, I guess a good idea would be to read about morphological operations. A very nice practical book is the legendary Digital Image Processing by Gonzales & Woods. There you will learn that the top-hat transform is built up from more basic morphological operators. Therefore, all the next lines give the same result:
ImageSubtract[img, Opening[img, 3]]
ImageSubtract[img, Dilation[Erosion[img, 3], 3]]
Additionally, you should be aware, that morphological operations probably don't work in all circumstances. Then, belisarius' approach might be an alternative (and worth an upvote) as long as the components you want to extract are separated.