auto reboot/program termination

My computer is about 4 years old but for the last few months it's been acting
strangely. First off, I usually leave it on because I'm on it frequently. I
also use MS Word avidly and leave the document I'm using open as well. The
problem I'm having usually only happens about once a month. I'll leave my
computer to do something else (my document is open) and when I come back to
the computer there is nothing open. My files are saved and up-to-date but are
not open when I specifically remember leaving them open.

I'm wondering if this is a glitch in the system from one of the LiveUpdates,
an early program termination, an automatic reboot (even though this option is
turned off), or something else all together. I do know that it is NOT a virus
as I scan my system weekly with Norton and my spyware program.

Is there anyone that can help me with this problem?