Toshiba Satellite A75 shut down randomly

[Acardi !!]

I am using Toshiba Satellite A75 since 2 years. But since past 2 days, it has started unexpectedly shutting down. Once started, the laptop works fine for first 15 to 20 minutes. But then unexpectedly without any prior warning shuts down. First I thought it must be the O.S. that has been corrupted. So I tried installing Windows XP again. During my 3 attempts, laptop shut down every time. I tried replacing thermal paste, checked the working of fans (which were working fine) but still the same. I once kept it on BIOS menu for around 3 hours and it never shut down. Can anybody help me?

[Ala-bama]

Toshiba A75 is very old, probably 7 to 8 years old model. So finding appropriate parts for it will be a big challenge for you. But as you say, it does not shutdown in BIOS mode and does during installation, then there must be with Hard Disk. I suggest that you should get your hard disk checked before proceeding to do any big change. If there would have been any problem with motherboard or RAM, then laptop would have shut down even in the BIOS menu.