This entire post is no longer applicable to the most recent version of VirtualBox. Here is a more recent post.

VirtualBox is well equipped for a Windows environment. Most of its features have a mechanism for automatic installation and setup. To get your guest OS, in Windows Vista, to have a real IP address from your router you first need a network bridge and tap. Open the VirtualBox manager:

VirtualBox manager screenshot

Then select a guest OS and open its settings. Then click the network tab:
VirtualBox manager network tab

Then click the add network tap button:
Add network tap button

It doesn’t matter what the name is :) Then adjust your guest OS networking adapter accordingly.

Enable host network

Now open your Windows Vista network configurations manager:

Microsoft Vista network configuration

This screenshot is what your network configurations should be after everything is done. To achieve this simply select both the VirtualBox tap, that was previously created, and bridge it to your LAN connection. Select both by click on each adapter while pressing the shift key. Then right click on either of the two highlighted icons and select “bridge”. This will automatically create a bridge between the tap and your real host networking adapter. Wham bam, now you can select a real static IP in your VirtualBox guest OS on a Windows Vista system :)

Update (12/4/2009); At the top of this post I say that the information here is outdated. You can view the post listed above if you want, but here are the pertinent screenshots also:

Here is a screen from Windows Vista. Go to the network tab of the virtual machine settings:

Network settings

Then select the attach to drop down menu:

Attach

Then change the attach to, drop down menu, to bridged adapter:

Bridged adapter