Voici l'explication fournie avec VirtualBox, mais ne conaissant deja pas Vista en francaisn en anglais j'ai du mal si quelqu'un pouvait m'aider...

4.2.4 Windows Vista networking
Windows Vista no longer ships a driver for the AMD PCnet Ethernet card that
VirtualBox provides to the guest by default (see chapter 6.1, Virtual networking hard-
ware, page 72). As a result, after installation of Vista in a virtual machine, there will
be no networking initially.
As a convenience, VirtualBox ships with a 32-bit driver for the AMD PCnet card,
which comes with the Windows Guest Additions. If you install these in a 32-bit Vista
guest, the driver will automatically be installed as well. If, for some reason, you would
like to install the driver manually, you can find it on the Guest Additions ISO. To install
this driver, mount the Guest Additions ISO (as described above, select “Install guest
additions” from the “Devices” menu). Then, start the Windows Hardware Wizard and
direct it to the Guest Additions CD where a driver for the PCnet card can be found in
the directory 32bit\Windows\3rdParty\AMD_PCnet.
Alternatively, change the Vista guest’s VM settings to use an Intel networking card
instead of the default AMD PCnet card; see chapter 3.7.5, Network settings, page 49
for details.