Mahoromatic

From Tokyopop:
Mahoro is a battle android created by Vesper to defend the Earth against alien invasions.
Having fought long and hard on behalf of Vesper, she only has 37 days left before she stops functioning. However, if they remove all her battle functions and equipment, Vesper can lengthen her life to 398 days. As thanks for her hard work, Vesper lets Mahoro choose how she would like to spend her remaining time.
She chooses to work as a maid for a boy named Suguru Misato.