weird usb problem

i am having this problem with my dell precision workstation 420...my usb just will not work for me, it's turnt on in the bios, and i have all my chipset drivers installed....the weird thing is if i plug say my mp3 player in it'l jump straight to unknown device but if i go into device manager and disable and enable the device again it'l work just fine...same with starting the system up with the device connected it'l work until you unplug it then u gotta start all over again.....anyone have any suggestions as to what might be causing this?