She really hits the evil button in the last three or four episodes of season 6.
Throughout seasons 5 and 6 there are flashes as well as a few moments in season 7. TV links is slowly adding more episodes of buffy as well as angel it does have most of season 6. Alluc is also occaisionally good.

giles has been warning willow abou magic since the beginging of season 3 but the evil willow starts when she gets in to rack with amy mid season 6. Sidereel.com has all of the buffy episodes