B737-800 CREW HAD THE CAPT'S SLIDING WINDOW COME OPEN ON TKOF ROLL.

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WE WERE TAKING OFF ON RWY 9 AT BOS ARPT. AT APPROX 85 KTS; THE CAPT'S SIDE WINDOW OPENED AND SLID TO THE COMPLETELY OPEN POS. AFTER 1 UNSUCCESSFUL ATTEMPT TO CLOSE THE WINDOW; I CALLED 'ABORT;' WE SWITCHED CTL OF THE ACFT (IT WAS THE FO'S TKOF); MADE A CALL TO TWR; MADE A PA TO THE PAX TO REMAIN SEATED; AND SLOWED TO TAXI SPD ON THE RWY. WE DECIDED IT WAS SAFE TO TAXI CLR OF THE RWY AND THEN STOP. ONCE STOPPED; WE COMPLETED APPROPRIATE CHKLISTS; CLOSED THE WINDOW (CHKED ITS OP TO BE NORMAL); CHKED OUR BRAKE COOLING CHARTS; AND THEN MADE ANOTHER PA TO THE PAX INFORMING THEM OF THE PROB AND OUR INTENTION TO TAXI BACK FOR ANOTHER TKOF. AFTER TALKING WITH OUR COMPANY OPS; WE ADVISED TWR OF OUR INTENTIONS TO TAXI FOR ANOTHER TKOF. THE SECOND ATTEMPT AT TKOF WENT SMOOTHLY. LESSONS LEARNED: 1) MAYBE MY LITTLE 'HIT' ON THE LOCKING HANDLE TO ENSURE THE WINDOW IS CLOSED NEEDS TO INCLUDE A SMALL PULL ALSO. 2) HAD THIS OCCURRED AT 120 KTS; I CAN SEE HOW THE INITIAL SHOCK MIGHT CAUSE YOU TO WANT TO ABORT. THINGS ARE ALWAYS DIFFERENT THAN THE SIMULATOR. AT 120 KTS; THIS NEEDS TO BE ADDRESSED IN THE AIR. 3) THE FLT ATTENDANTS CAN BARELY HEAR THE INITIAL PA TO 'STAY SEATED' DUE TO SEAT POS AND ABRUPT REJECTED TKOF MANEUVER. 4) STANDARD PROCS AND POLICY REALLY HELP WHEN 'SURPRISED' AT XA15 (EARLY MORNING) DURING TKOF. THE FO AND MYSELF FOUND OURSELVES QUICKLY PERFORMING THE PRACTICED REJECTED TKOF MANEUVER WITHOUT MUCH CONFUSION.