I have a pile of 2400's at work.
Well couple of things first, check all the connections in the case and make sure everything is seated good. If he had a video card in it, some 2400's don't have an AGP slot and if you used a PCI slot card the onboard vid has to be completely disabled for some reason. So yank the CMOS battery or reset the CMOS jumper.
To get the manuals for your machine find the quick service number on the case. It will be a black sticker with white numbers your looking for a 7 digit number. Take that number and go to
Dell's site enter the tag number you will get all the drivers and manuals for your box.