I had shutdown my computer and when I started it up again, I got a "FAN ERROR" at the top-left hand side of the screen.
That means the CPU fan has failed or is in the process of failing. You could try cleaning it (especially if you've never done that before), but if that doesn't help then a replacement would be necessary.
Is this a user-repaceable part and, if so, how difficult is it to replace the fan and what time frame should I expect (i.e. how long should it take, approximately)?
It is definitely user replaceable, and the replacement should take 15 minutes or less. You can see documents and videos showing fan removal and replacement here
. To see the videos click on "Take Course", to see the printed instructions click on "HMM" and go to the page in the Hardware Maintenance Manual showing fan removal and replacement. Let me know how it turns out, Scott.