I need to keep track of my opponents' opening repertoire.  My idea is that I will input their games by ECO code and therefore see what improvemnts they have come up with from game to game.  Is there a software that can help me build this database?

Most database software (including the free SCID) will let gobble up an entire PGN (1000s of games)'s worth of games and generate a MCO/NCO-ish opening table of tabiyas for you. SCID even has the veritable "opening tree window" with result/success %ages for each move as you progress down the tree.

thanks for the info!