Are you sure the fan just isn't being noisy because it's stuck on some dust or other particles? Try putting some compressed air through it. If you can disassemble it and get the fan in your hand, you can check if it's broken by simply spinning it with your finger. If the fan blades all stay the same distance from the base of the fan housing at all times, the fan is good - otherwise, it has become unbalanced or dirty.
You can always unplug the fan (whether or not your BIOS will allow the computer to run without it plugged in is anyone's guess). However, I would advise against using a USB fan to cool it - simply because the fan in your computer is controlled by your BIOS to make sure that your chip is not overheating. A USB fan would not carry that property.
Well, I sent my laptop in to be repaired because windows failed to start up. Which they then told me to buy their magic "boot discs" for 25 bucks or so. When I got the boot discs, they didn't work even after spending an hour and a half on the phone with their tech. So they then decided to warranty it. When I sent it in I also made a note to look at the fan because it was noisy. They didn't touch it. So the last thing I want to do EVER again is support Sony in any way. Their customer support is terrible and you can't get an American even if you ask for one...but that's neither here nor there.
I think I can get to the fan though. I had the keyboard and palmrest off last night and there is what looks like a protective steel shroud over the motherboard. I will try tomorrow and see if i can get the fan out.