Laptop shuts off when stacking on identical laptop

DO THIS ONLY IF YOU ARE COMFORTABLE WITH OPENING COMPUTERS!!!!!!!!!

If your laptop's fan is spinning this much, then most likely there is some dust stuck in the cooling vents.

Open up your laptop. Then, using a can of compressed air (usually CO2) simply blast all the dust and dirt out of the laptop, including the vents and fans. This should do the trick.

If that doesn't work, then clean the heat sink of the CPU. If that doesn't work, then remove the heatsink from the CPU and apply some heatsink paste. Contact Acer if the above doesn't work.
Update
Three remarks:
1) Prevent the fan from spinning madly while you use compressed air.
2) Before applying new thermal paste, first remove old old paste.
3) Thermal pasted is used in small quantities.

Many Dell laptops have a built in accelerometer to sense when a laptop is falling and can be configured to instantly turn off the hard drive to prevent damage on impact. It sounds like your accelerometer is too sensitive. I'm guessing there's probably a setting in the BIOS that will let you disable it. I'll do some research and post if I find anything specific.

UPDATE: Apparently there is a Freefall Sensor Driver. If you go to Dell support and put in your service tag, you can download the current driver.