Ok mates, thanks a lot for the infos, I'll try it right now and tell you if I'm ok with the solution.
edit : well, with my serial number, I can only download the Antivirus corporate edition 8.1.8.385a, or the 9.0.7. I can't remeber if I bought the 10.1.8, so I'll try to uninstall the version 8 and install the version 9.
re-edit : So, I've reinstalled the version 9 on the server (Antivirus server and System Center). I reinstalled the antivirus client on my clients (throught the shared folder VPHome). My clients are updated this time (virus definition 07/29/2009 rev.5).
As I still can't see the clients on the server with SSC, I tried to copy/paste the GRC.dat from the server (\prog files\symantec\GRC.dat) to the clients (\docs and settings\all users\application data\symantec\symantec antivirus corporate edition\7.5\GRC.dat), but each time I restart the client, the GRC.dat file is deleted, so the server still can't see the clients.
Any idea?