USB not detected on Droid or PC

Hi,

everytime i plug the droid onto my computer my droid will not charge or recognize that it is connected to the usb cord. When i plug the usb to the wall charger it charges fine, but it doesn't even charge or say mount usb when on the computer. I checked my computer, and all it does is make the noise saying usb device plugged in, and shows the little balloon that says installing usb device, but after that it dissapears, and there is no safely remove hardware. I was wondering if anybody has an idea on how to fix this.

thank you