Archived conversations and the 'show contact on desktop' options cause contact names and email addresses to be stored in the Windows Registry in readable format under HKEY_CURRENT_USER\Software\Patchou for the current user, and HKEY_Users/S-I-5-****/Software/Patchou. If you do NOT want other computer users to know who you are talking to when using Messenger, or which contacts you have displayed on your desktop, do not use Messenger Plus!!
That's the same thing like "If you don't want someone knows your high score for Minesweeper, don't let your friends use your computer."