Xander is attacked and turned. The gang finds out before he rises, and Spike's solution is to "override" the other vamp's bite and turn Xander as his own childe, while having Willow spell his soul back in. It works, and there are a copuple of days where Spike is holed up with Xander and Anya in their apartment, as Xander is working thru his awakening and getting his soul to be dominant over his demon. one of the details I remember is that Xander's chubbiness fades quickly with his now high "vampire metabolism". there is a shopping trip to the mall. I think Angel makes an appearnace, and has a confrontation with Spike, something about showing dominance over Xander as his grandsire.
it was a WIP when I read it, it may be complete now (it's been so long). I'd really love to find this fic again.