B737-300 CREW HAD THE SLIDING WINDOW OPEN AT THE START OF THE TKOF ROLL IN SJC.

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FLT WAS AN ORIGINATOR. SINCE THIS PARTICULAR GATE AT SJC HAS 2 ORIGINATORS; OUR ACFT WAS NOT AT THE GATE WHEN WE ARRIVED. WE HAD APPROX 15-20 MINS FROM THE TIME THE ACFT ARRIVED UNTIL PUSHBACK; WHICH WAS ON TIME. I WOULD IMAGINE THAT THE CREW THAT TOWED THE ACFT FROM THE REMOTE SPOT TO THE JETWAY HAD OCCASION TO OPEN THE CAPT'S SLIDING WINDOW AND PROBABLY DID NOT CLOSE IT COMPLETELY. I WAS NOT YET (I AM NOW) ACCUSTOMED TO CHKING THE SECURITY OF THE WINDOW AND OF COURSE BEING RUSHED THIS MORNING I FORGOT TO INSURE THAT IT WAS LOCKED. WHEN WE ADVANCED THE THROTTLES FORWARD FOR TKOF; THE INCREASED AIRFLOW/PRESSURIZATION CAUSED THE WINDOW TO OPEN WITH ALMOST NO FORWARD MOTION ON THE ACFT. WE RETARDED THE THROTTLES; ABORTED THE TKOF; AND TAXIED CLR OF THE RWY. AFTER LOCKING THE SLIDING WINDOW; COMPLETING THE PRETKOF CHKLIST; AND RECEIVING TKOF CLRNC; WE TOOK OFF WITH NO FURTHER INCIDENTS. IN THE FUTURE; I WILL NOT ONLY CHK THAT MY SLIDING WINDOW IS LOCKED; BUT ALSO BE MORE COGNIZANT THE CABIN IS PRESSURIZING AFTER ENG START.