The only time I've seen this, was on a motherboard that didn't have Port 60/64 Emulation enabled by default. Do you have an option like that in your DP55KG BIOS menu?
Thanks for the reply. I've had a good look through the BIOS and checked out all its nooks and crannies but haven't found that or similar. It does have the ability to turn on Legacy USB support without which the USB keyboard doesn't work at all. On page 63 of the DP55KG manual it mentions '7. Additional USB legacy feature options can be access by using Intel Integrator Toolkit.' but I've not investigated if this is a BIOS time feature yet, it sounds more post POST. Not sure why manufacturers have gone away from PS2, well, other than cost probably.