I had some windows 2000 and windows XP images i used in VMware.
I am now running virtualbox and booting them resulted in a Black screen with no response at all.
After a lot of rebooting, checking the agp440 solutions i found, i found the following website:
http://www.digizenstudio.com/blog/2008/05/20/vmware-to-virtualbox-migration-issues/
It pointed me to the extended features located in settings - system - motherboard.
Enabling the 'Enable IO APIC' setting is enough to get the VM's to start up in virtualbox.
After installing the guest additions of virtualbox, everything is running FINE !