There seems to be quite a bit of interest in fixing cordless drills.  Even I have done one instructable on the topic ( which I will put first here! otherwise no particular order ).  Most of the fixes involve the battery: some replace it, some soup it up.  This guide will help you find many of the instructables in one place.   If you have an instructable on this topic and I have missed it please let me know.  If you would rather junk the battery and add a cord see:  There are other things you can do with your cordless drill, more guides coming, one covers running the drill without the battery:  Cordless Drill Repair - Add a Cord