Have you ever messed with the insides of a computer? Adding another harddrive isn't difficult, quite easy actually. Before you do this however, is the Dell still under warrenty? Realise that if you crack the case to add the harddrive, the warrenty is now void.
So if your going to do this, this is what you do. Pull the 40GB out of any machine that its in. On the back where the power and IDE cable goes, look for a jumper. (there is a small chance that its on the PCB, but usually only scsi drives put their jumpers there.) If this drive was all by itself in the computer, its probably jumpered as master. If you look on the top of the drive, there should be some pictures as to how to jumper the drive as a slave. If not, look on the PCB (printed circuit board if you don't know what that means btw...) for something labeled Slave, S, Park, etc.
Open the Dell and look at his harddrive. (you might need to pull it.) Make sure its jumpered master. If its jumpered as CS/Cable Select, change the jumper on the 40GB to CS also. If you lack a 3 "headed" IDE cable, try hitting your local shop up for one. They probably have dozens laying around they will give you or sell cheap. The end all by itself goes into the motherboard, the middle end goes to the 40GB/Slave drive, and the far end goes to the master/90GB (90GB? Did they come in that size???)
Once thats done, plug in the power, and close the case. Power it back up, and partition/format as needed. Harddrives don't use a lot of power, so there shouldn't be any worries there. If you have ANY worries or questions, come back and ask first. (btw, the IDE connector should be notched so that it plugs in only one way. If it doesn't work, flip it over and try again.)