Okay, may I ask why? Is the point to emulate the human arm, perhaps to pass a captcha, or to avoid detection by an online game?
I've done this. As Nige says, you can use a little math to smooth the mouse motion. But there's a second way that uses less math (but not zero math.) You can simply record your own mouse movement, then "replay" that movement. All the "smooth, human and inaccurate" motion of your own hand will be replayed to the screen. All you have to do is scale the recorded motion to fit your simulated mouse motion. It's very easy, and when you see it in action, you will laugh like a giddy child who is getting on their first ride at Disneyland, especially when you place the macro in a loop that moves to random places on the screen. It just looks so human!
I have written such a macro, but it was over a year ago, and so I would need to rewrite it, because that's the way I roll. So I suggest that you take a stab at it today because I'm a little busy today.