Fixed when scrolling vertically, but not when scrolling horizontally

I am working on creating a spreadsheet type widget for a website I’m creating and I can’t figure out how to make the column headers fixed when scrolling vertically, but still allow them to scroll horizontally. An example of this is at
http://www.activewidgets.com/. Does anyone here know how to do something like that? I can keep something at the top like that using JS, but it looks all choppy. I just used a window.onscroll event, but it doesn’t look so hot. I think that this is probably just some sort of CSS trickery. Any ideas? Thanks.

-B