I just build a new system but now it won't boot. I can get into the BIOS but once windows 7 is starting it shows the logo and then I get a Blue Screen. The PC shuts down immediately so I'm not able to see the error code... I've only changed the mobo, the previous one was the ASRock z68 extreme 4.
If you didn't prepare your Windows install for the new hardware, then this is exactly what you can expect to happen.
Probably the quickest solution at this point would be to simply reinstall Windows. The primary alternative would be replacing the new board with the old one, hoping the OS will then boot still, preparing it for the new hardware, swapping the boards again, etc. That just introduces that many more opportunities for hardware to be damaged.
You can also try a repair install, but those are really not substantially different from a complete reinstall. Assuming you have a backup of important files (you do have regular backups, don't you), then it's probably easier to just format the drive and install from scratch.
first off go into the bios..check the mb bios rev..the newest is bios 1504. the shipping bios was 8xx. there few updates for memory and cpu code from asus for that mb. use the usb flashback port and a usb stick.download the bios rename it to z77st.cap and flash the bios. when powering up it take a min or two for the new bios to load you get a black screen down power off or reset the mb. when the new bios loads go into the bios and hit f5 save and exit. when the last bios 1504 is loaded and f5 is saved and you reboot go back in and under ai menu for dram speed set it to xmp profile. it read your ram and set it to the fast intel speed. when you reboot hit f8 go into safe mode. if it boots fine into safe mode go into window device manager and remove all of the drivers. (sound.video..mb chipset) with them gone windows should load at the next boot in normal mode and install the newer ones for the new mb.