sorry, don't know what happened to the rest of my comment above. I took it to the Apple Store and explained the situation, they ran their special diagnostic which told them that there was a malfunctioning temperature sensor in my computer, and the error code narrowed it down to my trackpad! Turns out when I was putting my computer back together I didn't properly connect the trackpad ribbon cable to the logic board. So all they did was open it back up, and re-seat the cable, turned it on and it now runs like a dream!
The computer was taking AGES and the fan was roaring up due to its own safety features, the computer detected a faulty temp sensor so it automatically heavily reduces the CPU clock speed (resulting in a VERY slow computer) and it spins the fan at max RPM as its assuming a worst case scenario of extreme temperature.
Hope this helps!! :)