Sorry I only know english enough to write, had to use google translate to read your question.
From my memory you should be able to start the lxde desktop by running the command lxdesktop. I would verify this in terminal but first you must turn off rox desktop. Menu --> Desktop --> Rox Pinboard on / off. Once it is turned off (no icons) you need to open a terminal and run lxdesktop.
If the lxde desktop apears you can make it permanent by editing ~/.icewm/startup to fit.
Control Center -> Desktop -> Edit icewm settings
Find and Comment out :
Then add :
save and close.
You will then need to set icewm as your default window manger (instead of rox-icewm) to prevent rox pinboard from starting. I am not sure why but rox pinboard is set to start 3 times in antix 12. once in ~/.xinitc (rox-icewm session), once in ~/.icewm/startup (icewm startup file) and once in the wallpaper scripting based on the session name (login_background if rox-* is detected).
Anyway to do
menu --> run
type: your root password
then when X restarts login and you should see the lxde desktop in icewm