I installed an external keyboard to my DELL Inspiron 1300 laptop, but I later unplugged and stored it away, however when turning the laptop on, a message appear "found a new hardware" and requesting instructions to install. How I can make it to "forget" there was ever an external keyboard?

Go into device driver properties and search for it and then disable it. You can always enable later on. Some software is probably hanging up. The other way to go is to let Windows go through the motions and when it gets to a point to where it cannot find the software or drivers just cancel out. It should give you a message asking whether you want to leave it like this, and click accept.