All I remember is he saves Buffy from Xander. Not much to go on I know.

thanks though

It might be this one.

What You Wished For by pfeifferpack

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=22222

Summary: Spike finds himself having a wish granted with unexpected results. The perfect opportunity for Spike to "get" his third slayer brings the vampire to a crossroads where he discovers a different goal altogether. Spike learns not only to be careful what you wish for, but also that when you swim in the Nile too long, you could drown. Starts in S4 at the beginning of "Goodbye, Iowa" then takes a quick turn to S1.