To do this you would want to write a program (on the PC) to listen for commands on the serial port from the Arduino and control the mouse. You would need to look into available libraries for this for whatever OS and whatever programming language you're using. The InterfacingWithSoftware page provides a large number of examples for various programming languages.
On the other hand, if you're just trying to build a sort of Arduino mouse then my answer stinks and you should follow HazardsMind's link. I'm assuming you still want to be able to communicate that analog reading to the PC.