First, you need GLaDOS's sound files. These can be found here: http://theportalwiki.com/wiki/GLaDOS_voice_lines (naturally, spoilers abound).
This site contains the audio files for every voice line in-game for every character in the Portal series, so if you'd rather someone other than GLaDOS (though why in Aperture would you?!), or want to supplement your new GLaDOS computer system with some other characters too, you can get their voice lines as well.
Next up, you're going to need an idea of what quotes you want to use, and get ready to set them. You'll need to open up your system sounds editor. On my Windows 7, I just right-click while in m