Toshiba Tecra A3 notebook problem

Hello everybody.
I need some tips or some answers, if you will be so kind.
First of all English is not my 1st language, so here goes nothing.
I just replaced a HDD for my Toshiba Tecra A3.
I installed Windows XP Home Edition.
Good. All worked fine. The next day i started my laptop, passed the XP logo, antivirus, wireless network all fine. Then it shuts down. I start it again. Same thing. After 5 times it works. And every day i must make so. I start the laptop, it shuts down, i start it again....5 times the ON button and then the laptop works.

Can you give me some advice ? i searched google but it always said it is the cooler. The cooler is clean, no dust..
Thank You