With some more testing, putting "options psmouse proto=exps" in /etc/modprobe.conf does not change. To hang the pc, I have to open some (five for example) nautilus windows and click upon the upper bar of each of them successively. Within 5-50 click, the screen freeze, and the keyboard is locked. Sometime, the applets at the bottom of the screen are still active. Booting with "acpi=off" does not change anything. Francois