k, are you familiar with adjusting them from scratch?

first, make sure the derailleur is completely square to the cogs.. tweak the hangar if not.

its easiest if you have a work stand. if its a 3x7, put the chain on the middle chainring in front, otherwise the big chainring. shift to the highest gear (smallest cog), and adjust the 'H" set screw on the derailleur so the idle wheels on the derailleur are centered on the outside edge of that small cog. now put the chain on the largest rear cog (if its a 2x, also shift the front to the small, otherwise 3x leave in middle), and adjjust the "L" screw so the chain is square with that socket when the derailleur is against the stop. you'll probably have to adjust the cable takeup, whever that is on your bike, to reach both extremes. now put the chain on the middle cog, and adjust the cable takeup so the derailleur wheels are centered on that cog. now ride the bike, middle gear, and shift up 1, down 1. if its slow going up 1, then tighten the cable a 1/4 turn. if its slow going down 1, loosen. repeat until perfect.

(thats all from memory sitting at my keyboard.... you can probably find better instructions with pictures at the Park Tools site)