Sudden shut down of Windows XP

I am running Windows XP professional on a Toshiba laptop that is barely
a year old. The computer was originally equipped with XP home. In a
haste, I upgraded to XP pro without totally reformatting the hard drive.
After the upgrade, this laptop would periodically cut off with no
warning. Since it seemed that the upgrade was to blame, I did reformat
the hard drive with XP pro. This fixed the problem until a week ago.
The newly reformatted machine ran 2 months with no problems. Now I
can be typing away and it cuts off with no warning. The battery has
been checked and seems to be fine. What am I missing?