I don't know much about talking computers but I'm thinking that there are only so many places a sound like that -- a whole word -- could possibly come from. If you were hearing a fragment of an audio or video clip, there'd be no reason for it to be a single neatly-defined word; you might just as easily get three and a half notes of music or something that sounded like "curSES!" accidentally clipped from "my speaker says".

I assume you don't have speech-synthesizing software on your computer (you type, it talks). How about other software that tosses out words for you to practice doing something with, like typing them? It wouldn't make much sense for it to engage by itself, though, never mind to do so only when you were at PC.

A stray part of an ad, maybe, left over from a multimedia sales pitch for a mattress store? A talking pop-under window? But again, I don't get the PC connection.

Have you scanned your computer for viruses, spybots and stuff, especially around the times when it's talked to you? If it were happening to me (on my older system) I'd make sure to check the "Task Manager" listing for anything unusual and to run HijackThis. I don't know which of these are available on (or can be downloaded to) your system, though, and I probably wouldn't be much help with interpreting the results.