arduino duemilanove loses connection


for school we have to work with the arduino, i have tested with an arduino uno. which was no problem at all.

today i have to work with an arduino duemilanove. it works fine in the beginning. but as soon as i add a sensor (ultrasone ping sensor) or a servo motor, my computer no longer finds the arduino, when i check my divices, on my com 4 the arduino is no longer there, i also can't find him then. when i remove the sensor or the motor it works just fine, on com4

greetz, Freaki

ps. my englisch isn't the best :S