The deceitful computer, GLaDOS (which stands for Genetic Lifeform and Disk Operating System), is the speaker and controller behind the Aperture Science Enrichment Center.
GLaDOS was created in response to the news that Black Mesa was developing a portal technology similar to that of Aperture Science. Her original purpose was to be an artificially intelligent research assistant (and disk operating system). Work on GLaDOS started in 1986, and was under development for more than ten years before she was finally completed several years after 1996. GLaDOS was activated for the first time, untested, during one of the events on Aperture Science's first annual "bring-your-daughter-to-work" day.
GLaDOS is made up of several "personality cores" in the form of strange spheres that apparently control the AI's behavior. Chell encounters a morality core, curiosity core, cake core, and anger core (in that order) while destroying GLaDOS.
Cave Johnson, founder of Aperture Science Center, in his wild pursuit of better portal technology, managed to contract cancer from the powdered dust of moon rocks. Over the years, he became more and more irrational until one day, in a fit of "genius", he decided to put his assistant Caroline in his new computer core. This was in the late 1970's. The final "version" of GLaDOS was not completed until 1996. The ten years it took to actually develop GLaDOS were mostly the OS portion.
The stresses of running the Enrichment Center were apparently too much for Caroline to handle. Her mind was simply not able to deal with the amount of testing the Enrichment Center needed to have completed on a daily basis. This may have been the reason for the addition of the OS to the computer core. It is unclear what drove the combined Human/AI mind to the point it is at when we first meet GLaDOS, but one thing is clear: the compulsion to test constantly certainly was a contributing factor. It is unfortunate that Caroline was the sane, stable part of GLaDOS. Caroline was also the only thing keeping Johnson from going completely insane.