Re-arm? Activate?
You go through the prompts and supply your valid Vista serial code when asked. Voila.
Oh that one.
'slmgr -rearm' isn't working? Are you doing it from an elevated command prompt? You have to run cmd as an administrator, so hit ctrl+shift+enter inside the command prompt, or right click and 'run as administrator' on the shortcut.
It can take a while to do it's thing before it asks you to reboot... Also you can only do it on day 29 or 30.