Well, I found the problem: The driver of a USB Wi-Fi device I plugged in caused the computer to crash.
Apparently, it's an old vista driver that's not suitable for Windows 7 64Bit.
Installing a different Wi-Fi adapter solved the issue for me.
You can check it as well if that's the case for you.
I recommend a software called "BlueScreenView" which tells you what file caused Windows to crash.