What if when putting in Spike's chip the Initiative gave him brain damage. He's lost all memory of his past and is now a clean slate.

Some how he comes across Buffy and Giles. One wants to kill him out right the other see's a good weapon to use. So they mold memory lost Spike into their perfect little attack dog. For some time he is very good as the Slayer's weapon.....becoming friends with the Scoobies...maybe a bit of flirting with the Slayer. 

But as most things go Spike's old memories slowly start to come back as the demon heals itself from it's injuries.