Does anybody know how to install driver to NVidia GeForce 9300M Gs on this longhorn build ? Windows XP drivers don't work for me ...

Try to install from the .inf files.
Do not run setup.exe but extract the contents with winzip or winrar or 7zip.

And go to the Device Manager search for the videocard, and then open it, and click update driver and then select it from the folder where you extracted it.

I try this, but after instalation of driver, I have only 4 colors and low resolution a it said to me that this driver is not supported for this OS, or somethink like that.

Maybe you can try to find and install a different version of the driver?

I didn't find another driver to my graphic card, I tried to update from Windows XP(with all drivers) to Longhorn build 3718, but on the start - Blue screen of death.

That is because you're windows XP = Service Pack 2 or 3, from 2004/2008.

And this build is based on Server 2003 RC code which was in the XP RTM area, so you need drivers that function on XP RTM (The original from 2001).
But this card is WAY to new to function on there because there are no drivers for it with XP RTM.

Try to install it on another PC, not on you're main PC, Microsoft also recommends everytime you install an Alpha/Beta/RC do it on another PC then you're PC for daily use or production use.
(Or is you only have 1 PC, try it in VMWare or Virtual PC, or if you really want to run this out-of-date and dangerous OS to go on to the internet (wich can't because of the time restrictions on the software) try an older videocard).

I tried to install Windows XP build 2600 but I have blue screen of death --- pci.sys .. So, Do you think that if I add drivers to the XP build 2600's ISO, it will help ? And after that, can I upgrade to the longhorn without the blue screen ?

The only working driver for the 8xxxM series on longhorn is this one:
http://www.laptopvideo2go.com/forum/ind ... opic=13667

Every newer driver gives 4 colors or gives a BSOD.
I think you are out of luck with your card, because this driver doesn't support it yet.
Try finding the oldest driver that supports your card and maybe you are lucky (search it on the forum i just gave a link of)

I don't understand it, windows xp drivers work good(on Windows XP). Does anybody know, why the windows xp driver don't work for longhorn ?

may because xp is not longhorn?