Excel Question: How do you make choices in one drop down list dependant on choices in another drop down list?

Example: Say you are buying computers; you have a choice of “Computer 1” or “Computer 2”. “Computer 1” only has a choice of a mouse or
Smart Card reader, “Computer 2” has a choice of a DVD-ROM or 17” Monitor.
In the first drop down menu, your choices of “Computer 1” or “Computer 2” appear. This choice determines whether the options of Mouse/Smart Card, or DVD-ROM/17” Monitor appear in the second drop down box.

Perhaps the best explanation of how to do this is Aladin's:

Dependent Dropdown Lists