Final update to this one after a long journey. Another new board arrived today and all is working fine with the Quad 8400. Turns out it was the board all along. Thanks to all for the replies and advice. 
I have an Intel Pentium 1.8ghz CPU in my Optiplex 755 SFF and am trying to upgrade the CPU to a Quad 8400. When i install the CPU and power on the fans run fast and i get a constant orange power light and nothing on the pc screen. The Bios is a22 which is the most recent. I have also tried a different PSU with the same results.  I have tested the Q8400 in a DEll 780 and it works fine.  I'm at a loss now to what may be the issue. I have read elsewhere that you need Bios A20 and not A22 in order to get a Q8400 to work but i'm not sure how correct that is.  When i replace the CPU back to the Pentium the system boots and Windows loads.  Any help here would be much appreciated 
Dell Optiplex 755 SFF Cpu Upgrade Problem