While there are many programs that allow PlayStation 3 owners to use their PS3 controllers as joysticks, the only program that allows the controller to be used as a mouse in Windows is Joystick to Mouse. Joystick to Mouse is free software that provides the user the ability to program buttons on the controller to perform specific computer keyboard and mouse functions, thus allowing for control of the mouse with the PS3 controller's analogue stick.
Download the Joystick to Mouse program from the link in the References section. Run the file Joy2Mouse3.exe when prompted, or save it to your computer's desktop, and double-click the file.
Follow the on-screen prompts to install the software. Then open the program by clicking the "Start" button on the Windows task bar and selecting "Programs."
Click the "Joysticks" option in the main window of the software. Then select "Profile Assignments." Make sure "Microsoft PC-joystick driver" is the option selected.
Click the "Profiles" tab, select the "Joysticks" drop-down menu, and select "Microsoft PC-Joystick." Click the "Apply" button.
Click the "Actions" tab, and set the mouse cursor as the left thumbstick. You can also change the acceleration and speed settings to control the sensitivity of the cursor. Click "Apply," then "Save" to exit the window.
Click the "Profiles" tab, and select "Actions Assignment" to assign functions to the various keypad buttons.
Unplug your mouse, and plug in the PS3 controller using the USB data cable that came with the console. You are now ready to use your controller as a mouse.