Hello Mauro,
I apologize that you are having issues with this. Installing anything on Vista for multiple users has become a serious complication. And even more difficult than that is getting Office addins to install with the correct .NET security settings for each individual user.
Outlook addins on Office 2007 require that they be installed PER USER. It is not possible to have it installed for all users. This means that the registry settings that tell outlook that an addin exists need to be configured for each user.
We have a configuration tool 'Configure TeamLook' that is installed in the Start Menu for all users. If the user runs this tool, it will copy all the necessary settings for that user so they will see TeamLook in Outlook. Unfortunately, in some cases, after Outlook opens, you may see the msi installer try to do something. We are working on a fix so this will not be shown, but for now you can just cancel out of this dialog like you have been doing. Everything will continue to work fine.
I hope this helps answer your question, and if you are still having problems, let me know!
Cheers,
~Joe