VirtualBox has the capability to add VMware .VMDK files into its Disk Manager. However, when launching a new VM from these, a few extra steps may be needed.

On the Settings | General | Advanced tab, in the Extended Features section, be sure the following 2 options are checked:

• Enable ACPI
• Enable IO APIC

Launch the Virtual Machine.

If that still doesn’t work, try this…

• For Windows XP, insert the Windows XP CD and make it available for the VM. Have the VM boot to the CD.
  To do this, go to Settings | CD/DVD-ROM | Mount CD/DVD-ROM Drive (Also, Enable Passthrough box checked)
• Launch the VM and have it boot from the Windows XP CD.
• Select R for Recovery Console.
• Type the number of your Windows install (usually 1) and hit Enter.
• Enter the administrator password.
• Type fixmbr and hit Enter.
• Answer Yes when it asks you to overwrite the MBR.
• Type fixboot and hit Enter.
• Type Exit, which to reboot the system and everything should work.