I believe you are understanding correctly.
I have Windows XP Professional SP 2
in a box which I bought years ago.
I installed it under Virtualbox 4.0.4 from
Oracle website. It runs on my Ubuntu 10.10 desktop.

I thought I would put the whole thing on my laptop.
So I installed the Virtualbox on my laptop
and copied the XP files from virtualbox desktop over to my laptop
and Windows XP runs just fine. Both update perfectly.

Of course I think Microsoft gave up on me because I activated
this Windows XP so many times in the past running VMware and
Virtualbox and dual booting and just playing around. :-)