Problem solved !
When a task was launch on Windows XP client : after it reboot on virtual partition, we got a "Error in EXE file"
an
d on server log we got a "error in file copy".
According to discussion : "One of the source files for the Virtual Partition could not be found" we have looked for ghost.exe on server and client.
We found on client that C:\Program Files\Symantec\Ghost\ghost.exe was 0 ko and on server there was no ghost.exe.
So we have deleted ghost.exe on client and copied a valid one on server.
Now all work fine !