Big Computer Problem

This is the problem: every time i try to run a 3D application, computer shuts down, or come back to Windows.
My computer configuration:
Intel Celeron 600MHz
Acorp 6A815EP1
2x 128 Mb 100 and 133 MHz
40 Gb Western Digital
350 W power source
nVidia Riva TNT M64 made by Jetway
I tryed both Win 98 and XP, and the same problem appears. Even in Age of Empires II, after 5-15 minutes of play, the game exits instantly and computer returns to Windows.
Some told me that the video card is old, but i tryed with other cards and the problem is the same. My video card on another computer works fine.
Others told me the ram is bad, but i tryed with another ram and was the same, and my modules worked on other computers.
Please help, i went to a computer service and they told me that it wasn't a hardware problem, but i don't think they were right. I used different versions of the drivers i needed, but this didn't fix the problem.
I'm waiting for your answers. Thanks.

I tryed all that in the first time. Thanks. But it didn't work....