Sliding menu help

Hello there,

I have searched hi and low for help with this issue.

I would like a part of my sidebar to scroll, like the scrolling media player on https://www.trackitdown.net

I followed this tutorial on Net Tuts

It was insightful, however it makes a menu which scrolls constantly.

Is there a way to make a part of my blog/section/div etc, only begin to scroll down when the screen reaches a certain point? Like the one on TrackItDown?

Any help massively appreciated! =)

Thanks folks!