Do every desktop computer has both video card and sound card built in?

Not exactly. The motherboard is the determining factor for whether your sound and video are built-in or not. Any machine from a large manufacturer (Dell, Gateway, HP, etc.) will have sound and video, but that doesn't necessarily mean that they are built-in. Sound and video cards can actually be peripheral add-on cards that physically plug into your motherboard through the use of specialized slots, or they can be the built-in variety which means they are physically soldered onto the motherboard itself from the motherboard manufacturer. The pros of having built-in components are cost (they generally cost less) and the opening of expansion slots for other add-on use (like if you added a firewire card or a separate hard drive controller). The cons are generally significantly reduced sound and video quality over their expansion slot cousins, and if sound/video ceases to work you'll have to replace the entire motherboard instead of that single component if the expansion slots are already ...  more