When I try to type on the keyboard nothing happens. The mouse works fine. I took an old keyboard from my desktop and plugged it in to the back of the laptop it works fine. But that is a pain. Help please

Might be a problem with the keyboard itself. Check your drivers for any problems. Also the event viewer (Start>Control Panel>Administrative tools>event viewer).

Try it in the BIOS and that way you will rule out a software/driver problem. I would say your going to need a new keyboard or a new motherboard. What system is it?

Dell keyboards fail often enough, particularly USB keyboards... but PS/2 keyboards also fail more frequently, just as a lightbulb can fail. We even see new Dell keyboards arrive bad in the box.
Since a replacement keyboard works, you can get a new one for $10 to $12, or a used one for $2. Repair shops throw hundreds away every year. Just ask for one, and you will probably get it free.
But before you install it, look for damage or crud in the socket.

The problem is with a laptop keyboard so it won't be usb.

I also got problem with couple of keys. After I reinstalled Windows XP, keys @ and " has interchanged. Few other sign keys also types differently, anybody got solution to my problem?