What to do?

I have a 'friend' (yes, only 1 ) who is such an a***hole.
He takes advantage of me all the time, he'll do stuff like steal money from me and even from my mum, hes making me so paranoid becuase everytime I accuse him of something he says its all part of my imagination. and it certainly isnt, I have bank statments documenting disappearing money.

Anyway, thats all in the past and I stand no chance of ever being reimbursed, but now the problem is he has my computer (not the one im on now obviously). I dont know how to ask for it back, hes taken out all my parts and replaced them with his and says hell give it back when he gets a case for his parts.
I lent him the computer because it was broken and he said he could fix it, then he did fix it but he said he spent 200$ on a new motherboard (which is for 'some' reason the exact same one that was already in there....duh). I know he is lying, because he has never done anything nice for me, EVER, so he is obviously making this up to scam 200$ from me, not to mention the fact that he STILL has my computer 2 months after he fixed it, and doesnt plan on giving it back. What do I do?? Im so close to actually killing this guy its not funny, hes driving me mental.
He humiliates me every chance he gets, he calls me an emo, a #######1, a bitch, and I do nothing but be nice to him. Not to mention he went behind my back and told my other friends stuff about me, and now I dont have those other friends.
Without a doubt when I get my computer back I never want to see him again.

Like I said, when I get my computer back I never wanna see him again. Hes a total sociopath/narcissist, but how do I get it back, whenever I ask he says ok ill give it back, but then procrastinates or makes up some excuse, like "i cant now cause i gotta pickup my girlfriend,' and theres always some excuse.