Jen Taylor
From Halopedia, the Halo wiki
Jen Taylor (or Jenn Taylor) is the voice of A.I. Cortana in both Halo 1 and Halo 2. She was born on February 17, 1973 in Seattle, Washington, USA. She is currently five feet six inches tall.
Jen Taylor is best known for her work as Princess Peach, Toad, Toadette and other characters in several Nintendo games since 1999, and the voice behind Cate Archer in No One Lives Forever 2. In 2004 she also appeared in the feature film Inheritance. She also has done voice-overs for the PC game Aliens Versus Predator 2 with David Scully.
It has been confirmed[1] that Jen will return to voice Cortana in Halo 3. Also she voices Jessica Cannon in the FPS; Sin Episodes.