[other] Copying a virtual machine from one OS to another

I run VirtualBox on my 64 bit Xubuntu 12.10 PC, and have created a WinXP virtual machine.

If I install VirtualBox on my MacBook, can I just copy the virtual machine files to it and run it?

Or does the virtual machine have to be created on the same kind of system that it runs on?

Thank you in advance.