How do you fix a locked touchpad on a Dell Insiron 1520?

I'm not sure what you mean by "locked". Will a reboot allow you to use it?
What symptoms lead you to think it is locked?

No I've tried rebooting it. The pointer won't move.

Do you have a mouse handy that you can plug in?

No. Do you know how to fix it?

What I would do would be to get the touchpad driver from the Dell site, delete the current touchpad driver and install the downloaded one.

Would I be able to do that with th touchpad not working?

Tough call. You'd need to be quite handy with keyboards commands. Easiest method by far is to plug in a USB mouse and hope it supersedes the touchpad.

Well I've just tried a USB mouse and its still not working.

Well then, you've got a little problem. You'll have to find someone who knows how to navigate the menus with a keyboard in order to delete the touchpad driver from Add/Remove Programs.