Agree with Arthur, Windows 7 should come with it, even if you press just Start and type Calculator in the search it should appear.
If for some reason it isn't there here is another calculator Microsoft do, I just tried it now, it's a bit weird looking but it should do the job, it even has a units converter on the left, and it also has the feature that the normal windows calculator has where you can switch from normal to scientific mode.
http://www.microsoft.com/en-us/downl....aspx?id=21622