MouseTrap is a free to use program which allows to lock the cursor only on one monitor in dual or multi monitor setup. It confines the mouse pointer to a single screen on multi monitor systems.
This program is portable in nature, So in order to use it simply unzip the file anywhere, and run it when required. Once running, ALT+Z traps the mouse in the monitor where the pointer is currently located, ALT+X releases the pointer and ALT+C exits the application.
This application is very small in size, it does not have any visual interface and does not even show any icon in system tray while its running. However it works on all versions of windows including windows 7, we have just tested the same on our windows 7 32 bit ultimate edition.