A DH8D CREW; DUE TO A COCKPIT WINDOW NOT BEING CLOSED PROPERLY; ABORTED THEIR TKOF FROM CGE; RESULTING IN AN EXCURSION ONTO THE RWY OVERRUN.

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DEPARTING NON TWR ARPT ON AN IFR FLT PLAN WITH A VOID TIME ASSIGNED; ON A PART 91 FERRY FLT TO PICK UP CHARTER PAX AT A DIFFERENT ARPT. THE FO WAS NEW; ON HIS SECOND FLT IN A HAWKER 400; BUT A VERY EXPERIENCED COMMERCIAL AND MIL JET PLT. THE FO WINDOW WAS NOT PROPERLY CLOSED ON PREFLT. DURING TKOF ROLL; THE SUDDEN AIR NOISE WAS NOTED AT 80-85 KTS AND I INITIATED AN ABORT AT 90 KTS. WRONG DECISION -- AND NOT ACCORDING TO MY PREDEP BRIEF IN WHICH I STATED THAT I WOULD NOT ABORT ABOVE 80 KTS FOR NON CRITICAL CONDITIONS. I DID NOT CORRECTLY IDENT THE PROB AND MADE THE ABORT DECISION THINKING THERE WAS A MORE SERIOUS CONDITION. THE ACFT WAS HVY BUT WITHIN THE WT AND TEMP LIMITS FOR THE RWY. PROPER ABORT PROCS WERE USED BUT WAS UNABLE TO STOP ON THE RWY SURFACE -- ROLLOUT CONTINUED 50-75 FT ON THE OVERRUN. TAXIED BACK ONTO RWY AND RETURNED TO MAINT FACILITY. HAD HAWKER QUALIFIED MAINT PERSONNEL INSPECT THE ACFT; BRAKES AND WHEELS AND THEN WAITED THE REQUIRED TIME FOR BRAKE COOLING. NO DAMAGE OR PROBS WERE FOUND AND THE ACFT WAS RELEASED FOR FLT. THE DECISION TO ABORT MUST BE MADE INSTANTLY; THIS TIME I MADE THE WRONG DECISION. THE HAWKER CAN SAFELY FLY WITH A WINDOW OPEN AND THE TKOF SHOULD HAVE BEEN CONTINUED. A PROBABLE CONTRIBUTING FACTOR WAS THAT I HAD JUST RETURNED FROM A 5 WK PERIOD OF NON FLYING AND MY PERFORMANCE AND SITUATIONAL AWARENESS WERE BELOW NORMAL.