After spending a few days on getting Windows 7 to shrink down the main partition so I could install Ubuntu on a new one, I finally was able to install Ubuntu from a flash drive.
The actual installation worked very well, but after rebooting Ubuntu didn't start but dropped me into a shell instead because of some UUID file missing. I booted from the flash drive again, chose "Try Ubuntu" and ran Boot-Repair. Suddenly I wasn't able to even select Ubuntu anymore. When I tried to run Windows, it crashed with a bluescreen and the repair mode and the magical three console commands weren't able to recover it. As far as I can tell, all of the files do still exist though.
When running Boot-Repair again, the Grub tabs are grayed out and "Repair Windows boot files" is as well. The summary is http://paste.ubuntu.com/5621035/ but after rebooting nothing has changed.
I hope you were able to follow me, after reading through different posts on this site and others, I still wasn't able to fix it so I decided to post here.
Edit: https://answers.launchpad.net/ubuntu/+question/221898 describes what happened and how I tried to fix it pretty well, although in their case it was Ubuntu 12.10 and not 13.04, which I use.