In this blog post, I have documented how to install minimal Ubuntu 10.04 in VirtualBox using netinstall. The good thing with netinstall is that the initial download is pretty small, around 13MB, but the total time to install the whole package takes a little longer.
If you have fast pipe, you can instead download alternate ISO. This link will download a copy from University of California, Santa Barbara. Using this ISO should get you up and running quickly into a fully working Ubuntu desktop.
Just for the uninitiated, boot VirtualBox guest machine using the ISO as download above. You should see a screen same a below.
After selecting the mode, press [ENTER] on "Install Ubuntu" selection. From here on, you should be able to use the instructions from here.