It keeps turning on!

[geuben]

My pc keeps turning itself on if its plugged in, but then it doesn't boot properly (fans spin up differently and there's no visual output). I've stripped it down to just the MB,CPU and RAM connected to the PSU and it's still doing it?

Any ideas?

 

[geuben]

I've taken it all apart and reseated everything, still turns on (but not properly) with only PSU plugged into the MB (no power switch connected)