Notebook logging off all time

Hi

I have a Win XP Pro notebook made by HP (good machine) and I have set the power options to never sleep, hibernation is off, because when its being used to do presentations it will not go to sleep. However, after a set period of time it logs me off back to the log-in option screen and I have to re-enter my password to reactivate it.

Can someone please tell me how I fix this annoying problem?

Thanks
Bruce