I have an XP installation on a virtual hard disk that I have been dragging around since I first started using Ubuntu.
For vitalization, I use VirtualBox OSE in Ubuntu, and it rocks. I love it. It's easy to use, with great features, and it is platform independent, so I've used it on Macs too.
But building new Virtual Machine on VirtualBox now sets the IDE HD interface as a PIIX4. This causes me a bluescreen error when booting my old Virtual HD.
Simply changing it to PIIX3 resolves this issue.