RaikuFA said:
I meant more like it breaks your computer and they go "sorry, you're SOL".
You'd have to check your warranty, but unless it's specifically the video card that broke your computer, then I wouldn't imagine it would void the warranty. And just opening your computer won't void it, only if you do something stupid and break it while your tinkering around.
And the only thing I can imagine a video card breaking would be your power supply unit. If it can't handle the wattage your new card needs, it'll brick the PSU, might take a while, but it'll go bad. Although installing a new power supply unit is pretty easy too. Takes a bit longer than a video card, but it's not complicated. But I think most of the PSU's I've seen in pre-built computers are like 400 Watts, and most video cards that I've seen need at least 500 Watts, so there's a good chance you'd have to replace it.
So yeah, long story short, as long as you don't screw your computer up, no it shouldn't void the warranty. So say you put in a new video card, and a week later the hard drive crashes, or your RAM goes bad, or whatever, as long as it's not related to the video card, which it shouldn't be, then you should be fine.
Just make sure to check the wattage of the GPU your looking at against what your PSU can supply, if your PSU doesn't provide enough power you'll have to replace that before installing a new GPU. But yeah, it's not complicated, I installed my first GPU when I was 12 and it went just fine.
Oh, and make sure to ground yourself before tinkering around, don't want to fry anything with static electricity.