FO WINDOW OPEN ON TKOF ROLL; TKOF ABORTED.

Narrative

TKOF ABORTED DUE TO FO'S SLIDING WINDOW OPENING ON TKOF. CREW WAS RERTED UPON ARR AT DFW; TO TAKE A FLT TO LAX DUE TO AN EQUIP SWAP. THE SUBSTITUTE EQUIP HAD BEEN UNDERGOING MAINT WHICH INCLUDED AN ENG CHANGE; FOR 3 DAYS. SINCE THE FLT TO LAX HAD ALREADY EXPERIENCED A 3 HR DELAY; ALL CONCERNED WERE IN THE RUSH MODE. THE BEFORE START CHKLIST WAS READ; 1 ITEM OF WHICH IS COCKPIT WINDOWS. THESE WINDOWS ARE CHKED BY ENSURING THE HANDLE IS IN THE FORWARD POS AND THE 'CLOSED' FLAG IS IN VIEW. THE FINAL PAPERWORK; WT DATA ETC; WAS RECEIVED AND THE FLT PUSHED BACK FOR DEP. SEVERAL DISTRS OCCURRED PRIOR TO REACHING THE DEP RWY. A NEW RTING BY ATC. INCORRECT PAX COUNT ON THE WT DATA RECORD WHICH REQUIRED COPYING NEW DATA SENT OVER THE COMPANY RADIO FREQ (2 PLT CREW). THE BEFORE TKOF CHKLIST WAS READ; 1 ITEM OF WHICH IS THE COCKPIT WINDOWS. DURING THE TKOF ROLL; I HEARD AIR NOISE COMING FROM THE FO'S SIDE OF THE COCKPIT AND SAW THE WINDOW HANDLE IN THE AFT; OR OPEN POS. I TOLD HIM TO CLOSE THE HANDLE; UT HE EXPERIENCED DIFFICULTY IN MOVING THE HANDLE AND IT BECAME APPARENT THE WINDOW COULD NOT BE CLOSED BY V1 SO I ABORTED THE TKOF. THE ABORT WAS NORMAL AND WE REQUIRED NO ASSISTANCE. THE FO LATER TOLD ME HE HAD NEVER OPENED OR CLOSED THE WINDOW. I THEN RECALLED THAT I NEVER OPENED OR CLOSED THE WINDOW DURING TRAINING. THIS WAS THE FO'S SECOND TRIP.