Why are there so few women in things like computer science and car racing?

I’m sure you can think of other examples where it’s a sausagefest, but really my questions are these: Why is it so? Is it a different set of reasons in each example? What steps can be taken to balance it?

This came up through two things I read recently in case anyone else wants to read them before responding:

http://www.topgear.com/uk/jeremy-clarkson/clarkson-women-2009-10

http://scicom.ucsc.edu/SciNotes/0901/pages/geeks/geeks.html