B767-300 CREW ABORTED TKOF AFTER THE CAPT'S SLIDING WINDOW OPENED AT THE BEGINNING OF THE TKOF ROLL.

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CAPT'S SIDE WINDOW BLEW OPEN DURING APPLICATION OF TKOF PWR AT APPROX 20-30 KIAS DURING TKOF ROLL. DISCONTINUED TKOF AND TAXIED CLR OF THE RWY TO MAKE SURE WINDOW WAS SECURE. SITUATIONAL DESCRIPTION: 1) FLT WAS AN EXTRA SECTION FLT ADDED DUE TO THE OVERBOOKING OF A PREVIOUS FLT. 2) MAINT WAS REPLACING THE R HEAT EXCHANGER ON THE AIR-CONDITIONING SYS WHEN WE ARRIVED AT THE ACFT. 3) ACFT HAD AN MCO FOR THE L BLEED SYS. HIGH-PRESSURE BLEED VALVE WAS WIRED SHUT DUE TO A PREVIOUS MALFUNCTION. 4) INTERIOR OF ACFT WAS VERY HOT APPROX 85 DEGS. DISTRS DURING PREFLT: 1) GATE BEGAN TO BOARD PAX WHILE WE WERE CONDUCTING PREFLT. I STOPPED THE PREFLT TO TELL GATE AGENTS TO STOP BOARDING THE ACFT DUE TO THE EXTREME TEMP. I WENT OUTSIDE THE ACFT TO HELP HOOK-UP EXTERNAL AIR-CONDITIONING. 2) RE-ENTERED ACFT TO CONTINUE PREFLT. MECHS ENTERED COCKPIT TO SIGN OFF LOGBOOK FOR HEAT EXCHANGER REPLACEMENT. I QUESTIONED MECHS ON THE L HIGH-PRESSURE BLEED MCO. A 'BLEED' LIGHT WAS STILL ILLUMINATED ON THE OVERHEAD PANEL. MECH SAID INDICATION WAS NORMAL. I TOOK HIS WORD FOR IT. 3) RAN THE BEFORE START CHKLIST. RECEIVED INCORRECT FUEL LOAD ON PAPERWORK. CONTACTED DISPATCHER TO GET PAPERWORK CORRECTED. 4) DURING THIS ENTIRE PROCESS WE WERE IN A HURRY TO GET THE FLT OUT SO PAX COULD MAKE CONNECTIONS. CONCLUSIONS: I BELIEVE THE ACFT WINDOW HAD BEEN OPENED AND CLOSED BEFORE WE ARRIVED AT THE ACFT DUE TO THE EXTREME HEAT IN THE COCKPIT. THE WINDOW AS CLOSED WITH THE HANDLE IN THE CORRECT POS; BUT WAS NOT FULLY SEATED IN THE LATCH DETENT. I DID NOT CATCH THIS ON PREFLT; PROBABLY BECAUSE OF THE ABOVE DISTRS. THE CHK WINDOW ITEM IS ALSO ON THE BEFORE-TKOF CHKLIST. DURING THIS CHK I PUT MY HAND ON THE WINDOW HANDLE TO MAKE SURE IT IS IN THE FORWARD POS. FROM THIS POINT FORWARD I WILL ALSO PULL ON THE HANDLE TO MAKE SURE IT IS LATCHED IN THE DETENT. I REALIZE I AM ULTIMATELY RESPONSIBLE. FROM NOW ON I WILL SLOW THINGS DOWN WHEN I FEEL RUSHED TO ENSURE COMPLIANCE.