Vostro 1500's problem when connected to an external display

Hello,

I have a Vostro 1500 and recently found a wierd display problem.

Normally I have used it home however I had a chance to use it for my presentation. So I brought it to the auditorium and connected it to a projector. However letters on the laptop's screen kept increasing and I could not stop it because key input was frozen. Finally my computer started making error sound (ding.wav) continuously. The only working solution for the rampage was turning it off.

After come back home frustrated (I did the presentation using another computer any way) I connected some other external displays to my laptop and the symptom occurred in the same manner. I have tried with HDTV, projector and monitor.

I reinstalled its video driver, however it did not make any positive change. If you have faced with the weird problem, please let me how to deal with it.

Thank you.