This afternoon I instaled a bigger hard drive in my PC.. Everything was going fine then about an hour ago it started going really slow and not loading any programs,, I did  reboot and still going slow, eventually froze and wouldn't even boot, I couldn't even get it going in safe mode.

I decided at this point it was the hard drive so plugged my previous one in.. I plugged the bigger one in as well to get data off but left the PC cover off

Here's where it gets weird; It decided to boot up all of a sudden and was working fine, so I decided it must have been the cables so I swapped them around.. Booted up fine with cover off and hard drive in place so I shut down and put the cover back on..

Turn PC on.. Won't boot! I take the cover off, boots fine so I put the cover when it has finished booting.. Freezes again.. Shut down, take cover off and boot up again... Boots fine!

What the hell is going on? The only thing I can think of is the caddy type enclosure my hard drive is screwed into and then clipped into the pc.. I thought I had put it in the wrong way but it only  goes in one way!

At the moment I have the cover off as it's the only way I can use my PC!