The ornamental design for a desktop computer case side panel, as shown and described.
FIG. 1 is a perspective view of the desktop computer case side panel of the present design showing the side panel installed on a desktop computer;
FIG. 2 is a front view of the desktop computer case side panel of the present design with a fan guard; and,
FIG. 3 is a front view of the desktop computer case side panel of the present design without the fan guard.
The broken lines showing of a part of the desktop computer in FIGS. 1, 2 and 3 are for illustrative purposes only and form no part of the claimed design.