I have that same problem when i bought it last year. Here's the solution to my problem.
The Acer crystal eye web cam driver have caused some problems with windows xp. So, i uninstall the acer webcam driver and even the application also. Any way, It works with the driver that comes with Windows XP. The shutdown problem is gone.... Just check if that is the same problem...... good luck.
I seem to have a similar problem but my window system is Vista.
My laptop works fine for 1/2 hours and just freezes every time. Does anyone have the same problem?
Any way around this problem?