Sometimes downloads, even legal ones, are corrupted which causes problems.
But in this case, Guild Wars is definitely trustable at all times, since they don't have a monthly subscription, they rely on purchases, and expansion packs, so they make sure everything is absolutely secure, and working properly in all cases.
Darmil, you should check the manufacturer's website. There's a list of supported video cards, and yours may not be one of them. That usually is the case. I know from experience =P