I performed the "3 fingers reset method" explained here in French:comment ca marche.
Stop the phone remove the sim card.
phone swtiched off, press the following keys together - all at the same time :
the key 
the key [*]
the [green key to pick up a call]
Press shortly the [turn on key] , lift it up again as ssoon as the phone vibrates and the nokia startup tune comes. keep the other three keys pressed until a prompt to change the date comes. My phone was reseted and the anoying memory problem was removed.
Wednesday, August 10, 2011
Nokia slide 6700 N memory problem
Had the problem, posted the solution here: nokia forum: