After spending some time on the net I finaly decided to go for a "online" transformation of my Windows XP Corp and OEM to a VLK (Volume License Key) one.
The only way I knew was to go with a repair of the installed version with a Corp VLK edition ... takes time and you have to redownload all the fixes ...

So tried to change few stuff and change the cd key ... and it works ;-)

Now I'm free of the M$ WGA (already installed on one PC by mistake) and it doesn't annoy me anymore ...

Follow the instruction in the howto.txt