Keyboard won't work in Finder/Desktop (but mouse does!).

So, I've got a Mac that boots up, works for a short while and then randomly the keyboard will stop working in the finder.

It will work in all other applications, but not the finder or desktop.

The mouse works in the finder, but not the keyboard.

It's a plain Apple keyboard and plain Apple mouse. No weird startup or login items.

I'd seen this mentioned somewhere in these forums a while ago, and was hoping someone remembered the fix?