The Ubuntu VirtualBox instance I use for Rails development can’t connect to the network unless you do some funky
sudo ifconfig eth0 up/down and
sudo /etc/init.d/networking restart magic.
I upgraded to Windows 7. Apparently, the Windows 7 adapter is buggy.
Thanks to this thread, all I needed to do was to change the adapter to “IntelPro/1000 T server” in the network settings.