As a software developer, when Parkinson's came knocking on my door it caused significant problems because I could no longer use a computer mouse to click on anything.
After searching the Internet I found to my horror that there were specialist computer mice available - with a price tag which was greater than buying a replacement PC!
So I figured that there had to be another solution which wasn't going to cost several limbs to acquire. Rather than explain all here, you might want to read the article which I wrote:
For less than $30 you can buy hardware and software which will make controlling a mouse cursor very easy, and whilst it is a little more inconvenient to use than a mouse you can point and click your way around a screen no problem.
Your local computer store will most likely have several different gamepad's on display. You need one which connects to a PC using a USB port (which your PC should have - if you haven't got a spare USB port then make sure to buy a USB hub to provide more ports). The only recommendation I can offer with respect to which gamepad to buy is 'THE CHEAPER THE BETTER'. You really do NOT need an expensive gamepad, and avoid any high-pressure sales guy trying to convince you that you should spend lots of money - for features that you will never need.
Some examples of gamepad's are shown on the following web page:
Note that these are only examples - there are a huge number of different gamepad's made by any number of manufacturers - your local computer store might not have the speedlink variety shown here, but would have something similar which would do just as well.
You should expect to spend some time configuring the JoyToKey utility to match the gamepad - although gamepad's all share a fairly common physical design, the electronics can map different buttons which send messages to the PC, and you will have to figure out the mapping plus what speed to allocate to buttons/joysticks. Some trial and error is required, but believe me it's well worth the effort to regain control of that damned mouse cursor!
Note: You don't have to throw your mouse (or trackball) away - these devices work quite happily if plugged in together, and you can switch between without having to reconfigure anything. As advised in the article, I use my trackball to whizz around the screen, then switch to the gamepad when I'm close to where I want to click.