If you want to replace an internal drive it simply a matter of:
Crack open the case
Take anti static procautions(either anti static wrist strap or leave plugged in but turned off and have a body part touching the case)
Remove the leads from the back of the old one and remove drive (note how the leads go)
Slide in new drive
The power cable will only go in one way so put this in first. You should see on your IDE cable that it has a red flash down one side, put this in so its next to the red lead on the power cable.
Then if you've got a lead to connect the drive to your sound card put this in this will normally only go one way too.
Close up the case and reboot
Hope that helps