Keyboard types extra letters

You could try cleaning the keyboard.
Sometimes debris gets stuck under the keys causing strange things.

Possibly the connecting cable is loose.

In truth, though, I think it might be time to replace the keyboard.

It's on a laptop, there's nothing under the keys, and it's been recorded on other forums if you do a quick search of the web.

Trouble is, all the links to potential solutions are dead. The oldest date I could find with exactly this issue is 2006. Same combo of extra letters and on multiple makes and models...

Get yourself a USB keyboard and try that. If the USB keyboard gives the same problem you have malware. If not then you're looking at a keyboard problem, possibly a new keyboard.

EDIT: There are known problems with keyboards, one is called ghosting where if you hit certain key combinations you also get additional keystrokes that have never been hit.