At 1530 mountain time, a Cessna 421C landed hard in gusty wind conditions at the Window Rock airport. The pilot and four passengers were not injured. The Part 135 flight departed Flagstaff, Ariz., at 1443 and made a VOR/DME-A circling approach to runway 02 in gusty wind conditions. He reported icing conditions with possible airframe icing as a contributor to the premature landing stall. Prior to the flights departure from Flagstaff, the destination weather was reported as 1,200-foot ceiling and 1-mile visibility, with a 900-foot ceiling and 1-mile visibility as the landing minimums. During the approach the local Automated Surface Observing System was reporting less than the weather briefing, but the pilot reported having the runway in sight.