They already had different MAC adress, surely changed by the sysprep I did when I created the image
Actually, no. Sysprep runs in the virtual machine and it's completely unaware that it's in a virtual environment - the virtual network adapter provided by VirtualPC appears like normal network hardware, where the MAC address is baked into the hardware and can't be modified.
Instead, if you try and add two VirtualPC machines to the VirtualPC console and they have duplicate MAC addresses in the .vmc files, VirtualPC automatically edits one of the VMC files to have a different address (if it didn't do this, the virtual network wouldn't work).
However, what it
doesn't do, but should do, is do the same for the SMBIOS GUIDs in the VMC files. In contrast, VMWare does handle most forms of this situation properly by recognising when you've copied the .vmx files around and asking you whether you want to replace the UUID or not.
This means that if you run VirtualPC and don't create new VMX files, you will probably have machines with duplicate SMBIOS UUIDs, and this means that you need the 2.0.1 update to GSS2 to copy with the fact that IDs which are required by the specification to be unique, aren't.
(should had clear the ghost client id too, no?).
There is no such thing stored in the client at present - although I have considered adding one as an option, Sysprep certainly wouldn't have a clue what to do with it. Instead, right now in GSS2 it uses the SMBIOS GUID or the network adapter MAC addresses to identify the hardware it's running on.