Do you have caliper brakes (that mount with a bolt above the top of the tire, adn the brakes arch over the tire) or cantilever/v-brakes (that mount on posts in the bike's frame, below the level of the rim)?
Given your mention of the screws, I assume you are talking about the latter.

Sometimes spring tension is different enough, either because of quality control or one of the springs getting bent, that screw adjustment isn't enough. Your options are
a) see if the bicycle frame has multiple holes on the brake-mount post where you can anchor the spring into the frame. This gives some incremental rough adjustment so you can tighten one spring or loosen the other
b) see if you can bend the looser spring so it's tighter. This won't necessarily work out well, but it's worth trying before
c) replacing the brakes (b/c it's hard to find springs by themselves and v-brakes are cheap)