My keyboard

Ah... I'm guessing it's not your keyboard, but your text editor.

It could be your keyboard, but I've never seen one act up like what you're explaining.

Can you describe the problem? What word processor you're using? Your Operating system? Laptop/desktop?

I've never seen the auto-spell function on a word processor before, or maybe I haven't wanted to look one up.
Microsoft Word does do a "false" completion to a long word. When you start typing a word, the processor will guess what you mean and highlight the rest of the word. But it doesn't fill in, it just shows you.

Agreed with WillSauger, this sounds like a word processor behaving badly.

Have you tried completely rebooting your computer?

If you are using a laptop with a touchpad mouse, it may be that the touchpad booted up with some strange sensitivity configuration that it interferes with the USB-plugged keyboard and mouse.

Also, you may want to check your USB connectors for a rogue wireless stud... LOL

I have a wireless Logitech K400 keyboard / mouse combo and installed the USB stud onto my kid's computer. Whenever he popped Diablo3, I could take control of his character remotely and make it do weird things. I did this several times over the span of a few days, all the while him thinking his computer was infected by some virus. So much fun.

-cb

ETA: The more I think of it, the more it sounds like you are using a USB mouse and you rest your thumbs on or near the touchpad as you type. Just deactivate the touchpad.

There are also various Operating System programs that could have booted up with a bad key combo that are affecting more than just the word processing program.