I claim the ornamental design for a handheld computer, as shown and described.
FIG. 1 is a left side elevation view of the handheld computer according to the invention;
FIG. 2 is a front elevation view of the handheld computer according to the invention;
FIG. 3 is a rear elevation view of the handheld computer according to the invention; and,
FIG. 4 is a right side elevation view of the handheld computer according to the invention.
The casing of the handheld computer includes finger alignment for purposes of identity authentication.
The broken line showing of the outer rectangle around the screen, buttons and keypad is included for the purpose of illustrating portions of the handheld computer and forms no part of the claimed design.