Hi Ethel,
I've never used a laptop in my life but it sounds as if your computer must have a built-in microphone. It's picking up signals from the speaker which it then feeds back to the speaker where they're picked up by the microphone ... ad infinitum. This is resulting in acoustic feedback. Plugging in the external microphone opens the connection, within the jack socket, which disables the internal mike. So, as long as the external mike is kept away from the speakers, the feedback ceases.
The solution would seem to be to either to disable the internal mike or to reduce its gain. There must settings somewhere within your Control Panel (or for your sound card) which allow you to do this. Unfortunately, since I use Windows ME on a desktop, I'm a little out of my depth at this stage ;-)
Anyway, Merry Christmas and many thanks for all your contributions to AB throughout the year, especially the ones which have corrected my errors ;-)
Chris