Most OS's today offer a way to use the mouse with the numberpad. This guide tells you how, since it lets you click way faster than you could with a mouse on its own! (unless you use an autoclicker)

No, old OS's are Windows ME and Windows 98, right? XP and 7 are still pretty new...

Open control panel, probably from the start menu

Enter the Ease of Access Center like this:
Hop into the part that makes the mouse easier to use, which can be done right from after step 2 like this:
Now, turn this on and adjust it however you want:

What the what?? Windows replaces old, perfectly functional features just beause they want it to look nicer?! It's more likely than you'd think.

Open the settings, from the start menu's bottom-left corner (above the power icon)



Scroll down the left sections until you find Mouse under Interaction.

Turn on Control your mouse with a keypad, and configure it how you'd like.

Once you have it turned on, all you need to do is use the numpad to click! [5] will click, [+] will double-click, [/] switches to left-mouse, [-] switches to right mouse, [0] starts holding the mouse, [.] releases the mouse if it's held and every number surrounding [5] moves the mouse in that direction, even diagonally!

On Windows only, you can make it so that [Ctrl] and [Shift] move the mouse faster and slower. It's not on by default and missing entirely in most Linux window managers.