>When installating SQL Server on Windows 7 64 bits, how can I be sure the installation will install the 64 bits version? When we click on Setup.exe from the root, it doesn't give us any choice. But, if I click on Setup.exe from the x64 directory, it tells me that I need to run the setup at the root.

There are different installation files - you need to pick up the correct installation disk if you're installing from DVD or download the correct version if you use MSDN subscription.