When connecting to my VPN using Windows 7 x64 in VirtualBox (on an OSX host), sometimes the connection will work but I still can;t authenticate with the Exchange server or the intranet sites. I found this depends on the router I am connecting to. For example, some coffee shops exhibit this behavior. Also when tethered to a cell phone hotspot, in my case a Galaxy Nexus on T-Mobile.
To fix this, I have to switch the Network Adapter setting from Bridged to NAT. What does this do? I have only a dim understanding. Magic, I conclude.