The ornamental design for a computer with an icon, as shown and described.
FIG. 1 is a front view of a computer with an icon according to the present design; and,
FIG. 2 is a front view thereof, with the icon shown separately for clarity of illustration.
The broken line drawing of the portion of a computer in FIG. 1 is for the purpose of illustrating unclaimed portions of the computer with an icon and forms no part of the claimed design.