Check your power settings.
I am using a Toshiba 1800-712 laptop. The link is : http://uk.computers.toshiba-europe.com/cgi-bin/ToshibaCSG/selected_product_option.jsp?z=267&service=UK&PRODUCT_ID=16699&DISC_MODEL=0
It is running on windows XP home edition.
The problem is that it frequently switches its self off and i am not sure why. It is using broadband with telewest and the modem is connected through the USB port. Also the computer is networked to a hub which is also connected to my sons computer. His computer is more powerful and is running on Windows XP Professional. My laptop is working as a gateway for him to connect to the broadband modem. The network is working fine. As i work from home, my laptop is on all day but it only seems to switch its self off when my son turns on his computer and uses the network. Could it be that the network is overloading my laptop or is it something else?