Alex, to remove the drive, follow the instructions from Dell :"
CAUTION: Before you begin any of the procedures in this section, follow the safety instructions in your Owner's Manual or Product Information Guide.
CAUTION: To avoid damage to the drive, do not set it on a hard surface. Instead, set the drive on a soft surface, such as a foam pad, that will sufficiently cushion it.
Shut down the computer through the Start menu.
Ensure that your computer and attached devices are turned off. If your computer and attached devices did not automatically turn off when you shut down your computer, turn them off now.
NOTICE: To disconnect a network cable, first unplug the cable from your computer and then unplug it from the network wall jack.
Disconnect any telephone or telecommunicati
Disconnect your computer and all attached devices from their electrical outlets, and then press the power button to ground the system board.
CAUTION: To guard against electrical shock, always unplug your computer from the electrical outlet before opening the cover.
Remove the computer cover.
NOTICE: Before touching anything inside your computer, ground yourself by touching an unpainted metal surface, such as the metal at the back of the computer. While you work, periodically touch an unpainted metal surface to dissipate any static electricity that could harm internal components.
Remove the hard drive:
Disconnect the power and hard-drive cables from the drive.
Remove the bracket-securing screw, and then remove the drive bracket from the computer.
Remove the three hard drive-securing screws, and then remove the hard drive from the bracket.
You can get the service manual for your computer from here.
As for destruction of your data, the shredding of a hard drive is universally recognized as the best method of destroying recorded data. The National Institute of Standards and Technology (NIST) Special Publication 800-88 "Guidelines for Media Santitization" identifies the shredding of a hard drive as the best method to assure absolute destruction of recorded data. there are a few more iideas out there like this one and here are further suggestions. Hope this helps, good luck.
Even though they're not exactly the same, the procedure for opening your desktop should be very similar to the Dell Dimension 4600. From there, you just need to disconnect the cables going to the hard drive and slide it out. This may require a Phillips screwdriver. As I've shown in the picture below, it looks like your hard drive will be at the bottom right against the front of the computer.