Somehow, Xander becomes Spike's. Spike takes out all his anger and frustration out on Xander. Xander apparently harbors some feelings for Spike and is devistated by his harsh treatment. He goes off in the woods somewhere and tries to commit suicide. I think it's somewhere of significance for he and Willow.
Spike finds him (how, I don't remember - Willow, maybe?) and is overcome with guilt for how he has treated Xander and vows to make it up to him.
Sorry I can't offer any more info. I usually remember more plot points. I do think that this one is at least a couple of years old, though.
Any and all help is greatly appreciated! Thanks.