Computer problems - Need advice

specs
320gig west digital hdd
amd phenom 965 x4
gtx 550ti
8gig corsair ram
msi 880gma-e35
corsair 700w

Since about... roughly two weeks ago my computer that i let friends use when they come to my place has been shutting down randomly.. ;(
I originally though that is was the powersupply but what makes me think that it is not the powersupply is the fact that the optical drive still operates when the cpu and graphics card are inactive, i can still open and close the optical drive but the gpu and cpu fans just don't spin, my case fans also spin, this lead me to believe that it was the motherboard, but i also think it could be the cpu or even the graphics card messing things up.. recently i also saw a slither of smoke come from the computer but i turned it off instantly and then turned it on about 10 mins later and haven't seen anymore smoke and i can't find any signs of burning, i took everything out of the case and inspected every inch of the components and i can't find any signs of burning but it was definitly smoke.. D:
What could be the cause? I only recently really got into computer around august last year so i'm still in experianced with the hardware side of how the computer works, i can build one and fix minor problems but this is above me, i checked my windows event log to see if it was software causing the machine to shutdown and i can't see any signs of the software causing the problem..

I'd appreciate any input you guys can give me!

Thank you,
Pachi