I'm using a laptop with built-in touch-pad that works in Windows 7. But after I switched to Ubuntu 12.04 LTS, it's not working now.
This seems to be a bug in 12.04.2. As stated in comments, a simple fix would be unloading the mouse driver and load it again like this:
sudo modprobe -r psmouse sudo modprobe psmouse proto=imps
This only keeps the touchpad working on this session. To make it permanent one can create a file like
/etc/modprobe.d/psmouse.conf (you can choose the name you want, but you must keep the extension) with the following content:
options psmouse proto=imps