Afterwards, some files are being copied to the partition...
... before the system gets rebooted:
In fact, it doesn't reboot, but shuts down. Go to View > Details:
Go to the Hardware tab and select Disk hdc (that's the Windows CD drive). Click on the Connect button:
Select ISO Image Location and fill in the path to the Windows XP ISO image (/home/winxp.iso).
Close the Virtual Machine Details window afterwards and then click on the Run button to start the guest again:
After a few moments, the Windows XP installation should continue. Just follow the wizard:
And finally, here's your Windows XP guest: