GIMP is a free program that you can download here: http://www.gimp.org/
When you have downloaded it, don't be scared by the interface. It's not too scary.
You essentially want to select the background and delete/erase it. In GIMP, that only works if the layer you are working with has an alpha channel. Once you have created the alpha channel, there are different ways to select the background, once you have done so you can then make the background that you have selected transparent, i.e. you delete that selected part of the layer. Make sure you save it as PNG file since that supports transparency.
If what I just said doesn't make sense, have a look at this tutorial and it should guide you through what you want to do: http://geekswithblogs.net/TimH/archive/2006/03/20/72797.aspx
I'm sure there are different ways of doing it, but that is just one way. Hope that helps!