Most likely, this is the CPU fan. After a while the bearings that support the axle become worn out and the fan starts to make noise, until one day it will stop working. The reason the noise stops when Windows starts is that the speed of the fan rotation can be controlled and it reduces the speed once all the heavy lifting of booting is done and Windows runs. Remedy: replace the fan or wait till it dies and buy a new laptop then. In the mean time I would use compressed air to clean out the duct and heat exchanger so that the fan does not have to kick in that often. B.t.w. when you do some heavy work Video conversion, Compiling, etc., the fan will likely kick in at a higher gear again and start squealing again.
Get a can of compressed air, open up your laptop, and spray all the dust out. – Enigma – 2013-02-25T18:56:03.670