Basically, if you have another slot near your current harddrive, then you just pop the new harddrive in.
You will need an IDE cable with two connectors. Look at pic...
Connect the new drive in the second port. Alos find an extra power cord that fits and plug that in.
You will have to check the drve manuals to see about jumper settings. The old drive needs to be set to master and the new one should be slave.
After that, you can go into Administrators tools in the control panel and then Computer Management then Disk Management. You can partition the new drive any way you like.
Voila! You now have 120 more gigs (or whatever).