In this story, Spike is in the basement and is bothering Xander when Anya arrives. They start to kiss to throw her off, but it turns into a real kiss. When Anya tries to hurt Spike, Xander throws her away and kills her accidentally. Xander is "broken" and Spike decides to take care of Xander as his demon now sees him as his mate. Later on in the fic, Xander regains some speech and tells Dru (who shows up) that she can't have Spike because he's "Xan's Spike." Thank you to the one that helps. I hope this is enough.
Found thanks to mckrycek. It was "Once Upon a Time" - http://www.angelfire.com/planet/rsk/arvs/onceaponatime.html