A computer with increased functionality

A computer (10), such as a notebook computer, can be upgraded to add a capability for receiving pen input data by the addition of an upgrade circuit module (42). Certain of the components necessary for the performance of this pen input function, such as an integrated digitizer (26) and display, and a portion of the circuitry for operating said digitizer (26) and receiving data therefrom, are incorporated into the computer (10) when it is manufactured and sold, but the pen input capability is not operable by a user of the computer (10) until the upgrade circuit module (42) is purchased and installed. An aperture (34) in the housing (28,30) of the computer (10) is provided to facilitate a simple user installation of the circuit module (42).