If you have a "Corporate Professional" version of XP, as I do, it never has
required validation by MS. The big users were not prepared to put up with
the nuisance of validation of every install so MS gave in and sold them a
special version. Somehow it has found its way, illegally no doubt, into the
hands of a lot of ordinary users.

Maybe MS doesn't care, but I suspect that if they could do easily it they
would prosecute.

The rest of your questions I will leave to the brains trust here.

Point 1) You have to install VBox again if you've done a clean install
of Ubuntu but the virtual machines themselves can be copied to an
external drive then copied back so you don't have to install from
scratch again.

Point 2) I've suspected for a while that the rules on activation of
WinXP have been relaxed quite a bit but if you copy your virtual
machine installs as above it'll not be a problem :-) Remains to be
seen what happens regarding WinXP activation after it officially dies
in the eyes of MicroSoft in 2014.

I dont have a prob as windows is banned from here :)