Christophe,
If I understand correctly you do not have the full Z600 HP Restore disc set. The Z600 came from HP with the rights to install W7Pro64, but many do not keep their Restore discs carefully filed, or do not know their value. What that Restore media does is allow you to install W7Pro64 using a universal HP OEM certificate of authenticity serial number (and provide the official software install to match) fine tuned by HP. Here is added value: a clone of that install will activate automatically on another same-type Z600. That is, a Version 2 clone will run on another Version 2 Z600. This can make life easy if one has a bunch of Z600s and you want to clone a fine-tuned enterprise build on them all.
Luckily most of the Z600s also came with a HP W7Pro64 COA sticker on the case. If you have a Microsoft W7Pro64 "system builder" disc you can build up from that, and enter that COA, and the build should activate (as a single-workstation build). Nothing you can clone from (unless you are willing to feed each clone a new individual COA serial number). You also should be able to find a French version of that system builder. I believe the COA will work fine for any language.
Regarding clean installs of W7Pro64.... they have become quite a pain. I only recently have begun to break through why the Windows 7 Update mechanism seems broken. It turns out that an automatic update to Windows 7 Update breaks Windows Update, but there is a workaround.... to be disconnected from the internet during a clean install from the optical disc (with service pack 1 on it), and then install two KB updates that takes Windows Update to a later version than the destructive one. This removes the trigger to go back in time and install the bad one. Then you can reconnect to the internet and Windows Update works fine thereafter.
MS is not currently forcing "upgrades" to W10Pro64, which is good. I am using that also, but some of our software needs to stay on W7 for now.
You'll need to get a system builder W7Pro64 SP1 disc if you cannot get an official Z600 version 2 HP OEM Restore media instead. If a friend has either of those it is would be legal to borrow as long as your Z600 was in fact sold with the W7Pro64 build or upgrade/downgrade rights to that.
p.s. I have gotten the Predator up and running on a xw6400.... just for fun, and it took some work, but I made the upgrade breakthrough in the process so the exercise was worth the effort.