Uninstalling Genieo broke my system
Turns out I couldn't access my disk when booting from disc (I apologise to all dyslectics reading this). I discovered, though, that since my last backup (which was 79 days before the crash) I had actually only used the computer very little, so I used a single USB stick to back up all files that were created or edited since then. (These files are fairly easy to find by sorting on last edited date in Finder. Luckily, I had still access to Finder.)
Booting from the disc in the Mac Box Set is indeed possible, I asked at the local Apple Reseller and did it myself. It was very easy (&lt;3 Apple). I just had to press C during startup, see http://support.apple.com/kb/ht1533 for the official list of startup key combinations. (That is where I found the information.) After booting from disc, I used Disk Utility to erase the disk and then reinstalled Snow Leopard. Fixed!