Breaking It Down
Once you have the power unplugged to your desktop PC, you're ready to open the case. Most computer cases have a side panel that comes off, but there are still some machines where the entire outer shell of the case slides off in one piece. Look for a couple screws on the back of your case, along the edges, where it might be keeping your desktop case from opening up. When you've located the screws remove them and place to the side. Gently slide the cover open. It might stick a little, that's okay.
Now that the cover is off, you can see all the internal guts of your computer. There should also be a sort of metal cage, protecting your hard drive. At the back, on the top, is the power supply. This is one of the most common pieces of a desktop PC to fail, rendering it inoperable. Take a good close look at it and locate the screws that hold it in place. Unless you have a bad power supply (PSU), you'll be leaving it in place.
Along the back edge of the case will be some screws holding your video card and any other expansion cards you may have, in place. Choosing the card with the most exposure, remove the screw. The circuit board at the bottom of the case is known as the motherboard. Some video and sound cards have release clips that will need to be activated before it can be removed. Check the base of the card, where it connects to the motherboard, for a release, before attempting to remove it. Detatch any cables leading away from the card. Once you're certain it's safe, gently pull the card from its slot. Don't wiggle and don't force it. One by one, remove all your cards in this manner.