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.