Installation not working with existing partition

[mikeoo]

I use a Synology NAS wit iPXE and i followed http://ipxe.org/howto/winpe instructions.

Al is working fine and Windows 8 install fine in VMWare.

But when i shutdown the VMware Windows 8 and try to start all over with an existing Windows installed the second time it is not working.

WinPE is booting fine
I can use net use to map to my NAS for the windows files and when i start SETUP nothing happens and after a while the windows (WinPE) prompt is back.

When i delete the existing partition with DiskPart i can start Setup again.

Why is the setup not working if there is already a partition. I don't want to delete the partition there but in Windows installation itself so i can save other ones like D: E: etc.