Displaying triangles

Hey! So, I’m drawing a land using TRIANGLES, lighting and color material but it doesn’t draw all of the triangles, it just draws like 90% of them. It’s not like it always draws the same triangles, it’s random.
I’m using an nVidia GeForce 8500GT. I have a laptop where I don’t have an nVidia video card and it runs fine, well it used to run fine until I formatted the computer, installed WinXP and got the latest drivers for everything, now it does the same thing as in my desktop (also with WinXP).
What can I do??? COuld it be that I’m using an old version of OpenGL to develop my software??
I opened GL.h and it says:

/* Version */
#define GL_VERSION_1_1 1

#define GL_VERSION 0x1F02

Get last error says?