DELL XPS 17 (L701X) video drivers not working on clean install

Hello, everyone once again.

I`m having a really hard time with my Dell laptop. After it fell to the ground some time ago, its GPU (Nvidia GeForce GT 445M) stopped working. As it is not possible to change only a graphics chip, I had to change whole motherboard. I ordered a refurbished one and installed it. As I wanted to make a clean install, I proceeded to driver instalallation, downloading the latest and installing them in the correct order (as shown on dell support webpage). All was working fine until I installed the graphics driver. After the restart my PC freezes onstartup - when Windows logo appears. In the beginning the logo animated but then just freezes and nothing happens.

Then I tried older graphics card drivers - the same thing. Even updated BIOS (maybe I shouldn`t have...) - still the same thing. Hardware seems to be working fine. Nvidia also detects the correct chip and proposes the correct driver. Tried also latest drivers from nVidia page, same thing. When launching in safe mode and disabling the video driver, PC works nicely with integrated VGA driver.

I also tried to launch W7 from my old HDD that I was using before my laptop fell - with all software and drivers installed and working fine at the time. When I launch Windows with GPU driver disabled, everything is working with integrated VGA graphics. But when I enable graphics driver - PC freezes on startup in the same manner. Same thing applies to the ubuntu - it freezes on startup. 

The guy who changed the motherboard assures me that it is not the hardware problem (I hope so too), but then I can`t figure it out where is the problem with the software. Can you help? Thanks!