Hi Roberto, 
Yes the uninstalled copy on the XP box needs to be deactivated. Simply uninstalling will not show up as a deactivation in our licensing server.  Unfortunately there is no way for you to be able to do that yourself on WinXP.  I talked to our licensing team, and if you send an email to sales@camerabits.com they should be able to handle this for you and make it possible for you to get get that XP install deactivated. 
Thank you, 
-Mick