Thanks man. These suggestions were nice, but did not solve the problem with the Windows install. I've checked the config file of the existing virtual machine too, and there was:
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2'>
<target dev='sda' bus='sata'>
Sata is not good for Win7? Strage for me. For bus type: ide, sata, scsi ... none of them works.
UPDATE: If I try to set both the CD-ROM and the HDD to ide in the domain.xml then the virtual machine doesn'T even start up. In the log file I can see this:
Error output: error: Failed to create domain from /var/lig/zentyal/Machines/Win7/domain.xml
error: internal error process exited while connecting to monitor: kvm: -drive file=/var/lib/zentyal/machines/Win7/HDD1.img,if=none,id=drive-ide0-0-0,format=qcow2: Duplicate ID 'drive0-0-0' for drive
Correct me if I'm wrong, but isn't he yelling about the fact that he gave the same ID for both the HDD and the CD-ROM?