Filling A Combo Box With Data From A Worksheet

I'm attempting to make a simple userform that inputs data onto an existing worksheet. I have the userform but would like to use a combo box to choose a "category" item of data, however I dont know what the categories are! I would like the combo box to, somehow, look at the spreadsheet and read off the already entered categories and offer those as choices.

A picture is worth a thousand words:
http://www.copestake.org/images/excel.png

Is there a simple way to fill the combo box (using the form initialize I assume) with the existing categories?

[Edited to link to image instead of displaying on board~admin]