After getting out the old computer and even the old tower case it looks like to me that anything that was in my new computer short circuited because the computer refused to boot up when I finally put in the old HDD drive I use for my OS and this is just by process of elimination. So what I'll do is put in a HDD I had lying around which was also untouched by the thermal paste disaster and that should finally boot everything up so I'll have a computer again, I just need to go and get a stupid HDD tray because the one I have has no screws for it and the screws I do have don't fit properly.

What was very interesting was that the bloody motherboard and everything all fitted properly, so it just goes to show I bought the wrong case, I had originally bought it because my knowledge about components amounted to "Need more space for big graphics card" and I bought it without realising I could have probably gone for something cheaper and also something that fitted all my components a lot more snugly.

When I think back, I'm pretty sure either during applying the thermal paste or when I was trying to fix my heatsink problem I did hear a spark so it probably was a short circuit that finally killed everything.