GL.ReadPixels failure on a certain equipment?

Hi all,

I developed a small tool for computing the object coordinates of a certain pixel picked by the user. I used GL.ReadPixels to read the depth component of such pixel, in order to obtain its Z component in window coordinates. Then, I use gluUnProject to compute the world coordinates. It worked well, no problem.
The problem came when I discovered that my app worked in all the PCs except on a certain laptop. After comparing results of exactly the same operation in the same conditions and in several PCs, I found out that the return value (winZ) of GL.ReadPixels was always the same except on such laptop. Therefore, the error seems to be there.

GL.ReadPixels<float>(winX, winY, 1, 1, OpenTK.Graphics.OpenGL.PixelFormat.DepthComponent, PixelType.Float, ref winZ);

The question is if anyone could know... how can this occur? Why different result for GL.ReadPixels on a certain equipment?.

The equipment specs are:
http://uk.computers.toshiba-europe.com/innovation/product/Satellite-P850...

Many thanks!
Happy 2013