just buy a new one, should get yourself a dvdrw. (they burn cd's too) but for the same price youd be crazy just to get a cdrw.
and its simply a case of taking your case off, unscrewing the 2 screws that hold the drive in, unplug the 4 pin plug, unplug the thin IDE cable.
look on the back of your cdrw and see what the jumper is set to (master/slave/cs-cable select) and put the jumper on your new dvdrw the same way. plug in the ide cable, plug in the 4 pin, screw it back in, done.