Alright, I made a little search about my problem but nothing seemed to be really solved. At least I didn't found the solution yet. Actually, I just installed the graphic card Quadro 4000 (the one for Mac) on my mac pro 4.1 with OS X 10.6.8. I first installed the drivers on the Mac side + the Cuda drivers thing. I booted on the Mac side and everything seems flawless, even faster than before! Only issue is, when I try to boot on the Windows 7 side, right after I selected the Windows side to boot and pressed "Enter" it freezed instantly. I didn't installed any drivers on the Windows 7 side, so I guess it's the issue. I went on the NVidia website to download the drivers for the "Quadro 4000 for Mac" but it tells it's not available for Windows 7 (obviously, since it's a Mac Graphic Card). Thing is, I don't want to try anything before being sure of what I'm doing so I ask here, what's the exact problem? Is it really the lack of drivers on the Windows side, if yes, which drivers should I download on the Windows side? (The Quadro 4000 for Windows drivers?) Thanks for the help, would be highly appreciate!