The green light is on your floppy disk drive it looks like. It is trying to access the drive for some reason. You could remove the drive (just unplug power and ribbon cable) and see if it fixes it. I highly doubt you use it anymore anyway.
You should probably figure out what is trying to access it. There might be an easy way to do that, but you can find out for sure in your event logs because it will be tossing errors or warnings like mad.
edit: Actually, at second glance it might be a card reader there. It is hard to tell on the pictures on the dell site so I found some bigger ones.
Is this machine the same configuration as yours?
Is the green light on the same panel as your card reader or is it on the panel below that, inbetween the audio jacks and USB ports?