Why does my laptop do this sometimes and how can I fix it?

It’s a crappy old Dell Latitude D600. Every couple of days or so, I’ll be typing and clicking as normal, and suddenly I can’t click on anything. The cursor will still move, but it doesn’t allow me to actually click. Nothing happens when I try. BUT- if I click off of the screen, onto the desktop, it’s fine for a few clicks. Then it goes back to not being able to click. Then, it’ll go back to normal after a while.

Any suggestions? Thanks.