WHILE EN ROUTE, THE ARICREW ENCOUNTERED STRONGER HEADWINDS THAN EXPECTED. THEY REALIZED THERE WOULD NOT BE ENOUGH FUEL RESERVED FOR AN IFR FLT TO ASPEN, SO THE ELECTED TO DIVERT TO EAGLE, CO. HOWEVER, THEIR LANDING AT ENGLE WAS DELAYED DUE TO A SNOWPLOW (WITH NO RADIO) ON THE RWY. AFTER CIRCLING & REENTERING FINAL APCH, THE AIRCREW NOTED THAT THE SNOWPLOW WAS STILL ON THE RWY, HEADED IN THEIR DIRECTION. THEY CONTINUED THEIR APCH & THE SNOWPLOW CLEARED THE RWY. ACCORDING TO THE SNOWPLOW OPERATOR, THE ACFT WAS 100 TO 200 FT AGL WHEN IT CROSSED THE THRESHOLD. A FSS SPECIALIST NOTED THAT THE ACFT WAS STILL AIRBORNE WHEN IT PASSED THE 2ND TURNOFF, APRX 2300 FT FROM THE THRESHOLD. AFTER TOUCHING DOWN ON THE SNOW COVERED RWY, THE PLT DEPLOYED THE SPOILERS & IMMEDIATELY APPLIED BRAKING ACTION, BUT WAS UNABLE TO STOP ON THE REMAINING RWY. THE ACFT CONTINUED OFF THE END OF THE RWY, WENT DOWN A 15 FT BANK & RAN INTO A SMALL RAVINE. THE RWY GRADIENT WAS 1.4% DOWNHILL, ELEV 6538 FT. DRAG CHUTE NOT DEPLOYED, 600 LBS FUEL RMNG.