The latest version of Virtualbox 4.04 doesn't allow you to directly add a VMWare disk in the Virtual Disk Manager, like it used to in Virtual box 2.x .
To get it to work, you need to create a new VirtualBox image and then, choose "use existing disk" and use the little yellow folder icon/button to browse for the .vmdk and choose it.
1. From within VMWare, Uninstall VMWare tools before you do this.
Otherwise your in for a world of hurt trying to manually uninstall it and
Windows XP will deny detecting the network hardware in the VirtualBox
environment. Also, download the Intel network card driver package to
your VM (required by VirtualBox) before you move it away from VMWare.
2. Make sure that you choose the right motherboard chipset and # of
processors in your VirtualBox config, otherwise the disk might not boot.
3. Be careful to choose the right .vmdk image file or else it might seem
like the disk doesn't want to mount while within the wizard. The smaller of
the 2 files is probably the one you want, as that one seems to be a map to
the larger .vmdk file.
Intel Drivers: http://www.intel.com/support/network/sb/cs-006103.htm