No, you don't have to re-activate.
You must remember, though, that a DI image will only install into EXACTLY the same computer setup as the one it was made from. This means that you cnnot use the image for cloning into second, third and more machines.
There is normally no need to clear out your machine as frequently as you say you do. As long as you do regular housekeeping and keep junk to a minimum on a day to day basis, drastic steps such as you do are not necessary.
As far as re-activating XP is concerned, you have a number of opportunities via the website before you need to telephone them. Frequent re-activations by phone will arouse suspicion and you may get quizzed as to what necessitates this.