Salut Cédric.
We need to know the version of Zimbra you're using (on which OS) and the exact version of the Outlook 2003 that are running on the W2K computers.
We also need to know which error you're getting and the ZCO log would be nice too (see here :
Outlook Troubleshooting Options - Zimbra :: Wiki ).
If you have problems with answering in english, answer in french, I'll translate.