i finally decided to perform a good-old Windows reinstall (for other reasons, too), so the problem is gone.
The details of my issue, though, were :
The Windows security screen was appearing when I hit Ctrl+Alt+Del, but with the following greyed :
-Log Off
-Task Manager
-Shut Down
At start up, i had to type my login/pass, if it's what you call the welcome screen
I could not access the Task manager with Ctrl+Shit+Esc, getting a "Task manager was disabled by the dmin" message or alike.
Thanks anyway for your help.