Basically it's not hard at all.
The question is: Is it your intent to have two hard drives? Are you keeping the 40G installed or are you removing it and only using the 120G?
If you intend to install the 120G as a 2nd drive, it is really simple.
First of all, you need to open that Dell up and see it there is currently a physical place for the 2nd hard drive. The Dell I just bought my wife requires that you purchase an additional mounting bracket from Dell before you can install the second drive. This is typical of Dell, HP, Compac, etc., etc. They simply design their cases so you have to buy parts from them.
Assuming your going to have 2 hard drives and there is a place for it do the following.
CRIITICAL: Look at the back of the new hard drive and you will see a small jumper. Look at the jumper configuration figure on the label of the HD and pull the jumper off and reinstall it to set the new HD to "slave".
Open the PC up and install the new hard drive next to the current one. You should receive mounting screws with your new HD.
Find the flat grey IDE cable that currently goes to your current hard drive. There are two black connectors on the gray IDE cable that are close to one end and they are about 6 inches apart. The black connector closest to the end of the IDE cable must go to the master HD which is the one that is currently installed. The black connector that is about 6 inches back along the cable will go to the new HD.
Find a spare power cable leg and plug in the power cable onto the new HD.
Boot the PC up. Windows should recognize the new hardware and will effect the Windows install of it.
You may want to go to the Western Digital web site (www.westerndigital.com) and download their software for easy formatting of the new HD. Yes - it must be formatted. If you are using XP make sure you choose NTFS file format.
Once that is done you should be all set to use the new drive. It will be Drive D and your CD/DVD drives will be E, F, etc.
Hope this helps.