DON'T BLOCK THE AIR VENTS!
Many people put their laptops on a soft surface (like a bed) and wind up smothering the vents and basically not allowing the cooling fans to do their job. Depending on how you hold it in your lap and how soft your legs are, the same thing can happen there. That is why when I put mine on my lap, I place it a couple of inches to the left so that I don't block the vent with my thigh.
To get an idea of what that is like, take a pillow, hold it firmly over your face, and then try to breathe. What? You can't move any air that way? Well, neither can your laptop fans, so the heat sinks that rely on getting air circulation to prevent the chips from burning up can't get rid of the heat.
The fans get loud because they are trying to move more air by running faster; higher motor speeds mean more noise.
As for the freezing, that is your systems way of saying, "HEY! I'M FRICKING HOT!". Cars shoot steam out from under the hood, mamals pass out from heat exhaustion, and PCs either freeze or (if adequately sophisticated, like my desktop) shut down.
Basically, rest your laptop on a hard surface when it's not in your lap and make sure you're not blocking a cooling vent when it is. Those little slots in teh case are not just there for decoration ;)
