Installing Windows7 turned out to be not so easy as previous versions. Booting the DVD within a VirtualBox virtual machine barfed with:

  CDBOOT cannot boot from CD code:5

The solutions on the interwebs mentioned something about ETFSBOOT.COM or using Gujin (a boot loader) to boot from the CD. But the thing that worked for me was:

Boot a Windows Vista Recovery Disc
Select "Repair my computer"
Select "Command prompt"

At this point we should be able to remove the Windows Vista DVD from our virtual machine and replace it with the Windows7 DVD. On the command prompt in the virtual machine again:

Select your CD drive, e.g. type "D:" if your drive is listed as drive ''D:''
type "cd sources"
type "setup.exe"

Now Windows7 setup should come up. The Windows Vista setup window might still be active and can just be minimized.