Pre-chip Spike seduces Xander, seduce might even be to nice of a word for it, but they had sex. I think it was a sort of Lost Weekend, because Spike gives Xander the option of taking a magic pill to forget their nekkid adventures. Spike assumes that Xander took the magic drug, but years later finds out Xander didn't want to forget because Xander still has the magic pill thingy (I want to say it was a cherry like object?) I think Spike finds this out Post-Chip, maybe even Basement of Doom sort of thing.
p.s - Happy Halloween!