He is an Imperial "Ground Combat Trooper" from the very first set of Rogue Trader minis. He is one of the original 20 or so minis that were ever created for the 40k universe. This guy came from the same set as the legendary Adeptus Custodes model. He is an ancestor of the Imperial Guard Storm Troopers.
That's a pretty old and rare model you've got there.
Ghazkhull