Question on Drop-down List in Excel

In Excel 2000, User can create a drop-down list from a range of cells, for example, “Europe”, “Asia”, for Column “Continent”, so that operator can only select from the predefined list. Now we want to create two columns, for example, one column “Continent”, another column “Country”, hope to realize the function as: once operator select “Continent”, let?s say, Europe, the Country column list only show UK, Germany, France, etc.; when he select Asia, the Country list only show China, Japan, India, etc.
After I select the first column, it will update the source for the second column in the same row.
How can I realize this? Thanks.