Force Preventing PC from system initiated shutdown

I own a Compaq Presario A906 laptop (which i like a lot, especially its 17" screen) that is 4.5 yrs old & running on XP. Recently, on three separate occassions, it would just shut down by itself almost immediately on boot-up. Checked no virus, and all system files are updated. Reading through the forums, I believe this is caused by an over-zealous protection system linked to heating up of the vents. I am confident of my machine and would like to prevent any future uninitiated shut-down. I came across these programs: Amphetamine, Don't Sleep and Insomnia but believe they are more of "better interfaces" for the system shutdown/sleep profiles under Power Option control panel rather than truly ensuring no shut down (ps: my profile is set to "Never" sleep or shutdown). Does anyone know of any software that can force the machine to NOT shutdown unless initiated by the user ? Maybe the softwares mentioned above may be able to do this - any experience ?

Thanks for all help in advance

 

PS: just to clarify - the machine justs shuts off, there is no shutdown procedure. It is like the power plug has been removed and there is no battery in it.

 

Have you actually checked the temperatures of all on board hardware? Are all the fans operational and have you cleaned out all the vents. Most occasions that I see what you describe is caused by failure of the CPU heatsink fan. (They're also usually Compaqs)