Hi amira,
if I understand well the information you’ve posted, the windows_xp image contains an ISO image. It that is the case you should edit the type that is currently set to OS and change it to CDROM. Then in your VM template in the OS booting section you would select the CDROM as the first device to boot. You may also need an additional DATABLOCK disk where you can install the operating system (you should first create the datablock as empty and then you would set it to OS).
Here are some links and useful posts in the forum that may help you:
- http://docs.opennebula.org/5.0/operation/vm_setup/add_content.html#install-within-opennebula
- How install windows vm?
- How to create Windows VM?
Hope it helps you!