How do I create a list within one cell

I have a worksheet where a number of functions are dependant on the contents of a single cell. I have created a multiple IF function which fits my conditions however if the cell is left blank or spelled incorrectly then the function does not work. A simple list and drop down menu would solve this problem, but I need it to be contained in one cell as the adjacent cells are occupied. I'v seen the one cell drop down list in other worksheets, just wondering if anyone knows how to create one

Look at this video. The second half contains information on drop down lists.

http://www.datapigtechnologies.com/f...alidation.html

Alan