I think it was called 'Spike's got mail', or something similar. Spike and Xander become friends through e-mail but dont know that they're speaking to each other. Then Xander describes to Spike the guy that he likes and Spike realizes that it is him and realizes that it's xander that he's talking to. I think they were both in the same building when this happened but in separate rooms.
I'm not entirely sure on the details, I read it ages ago.
Thanks in advance.