B737-800 CREW HAD ERRONEOUS WT AND BAL CALCULATIONS AFTER A REPLACEMENT LAPTOP WT AND BAL COMPUTER WAS NOT INITIALIZED PROPERLY.

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OUR WT AND BAL/PERFORMANCE COMPUTATIONS ARE PERFORMED IN THE COCKPIT ON A LAPTOP COMPUTER WITH 'ONBOARD PERFORMANCE SYS' SOFTWARE INSTALLED IN IT. ON THE DATE ABOVE; I WAS NOT ABLE TO BOOT UP THE COMPUTER AS IT WAS MALFUNCTIONING. I RADIOED MAINT AND THEY BROUGHT ANOTHER COMPUTER TO THE ACFT. I BOOTED IT UP AND COMPLETED OUR CALCULATIONS; AND WE PRESSED ON. JUST PRIOR TO THE THIRD FLT OF THE DAY; I REALIZED THAT I HAD NOT SET OUR ACFT NUMBER INTO THE COMPUTER AND THAT WE HAD BEEN PERFORMING OUR CALCULATIONS WITH ANOTHER ACFT NUMBER SET. THUS; THE BASIC OPERATING WT AND CTR OF GRAVITY ENVELOPE WERE SLIGHTLY DIFFERENT AND THE RESULTS OF OUR CALCULATIONS WERE A BIT OFF. THE PRINT-OUT OF OUR CALCULATIONS SHOWED THE WRONG ACFT LISTED AS SET. IN COMPARING THE 2 ACFT; I FOUND THAT THE ACFT WE WERE OPERATING WAS 382 LBS HEAVIER THAN THE ACFT USED IN OUR COMPUTATIONS. NO MAX CERTIFICATED WT LIMITATIONS HAD BEEN EXCEEDED; BUT REDUCED/DERATED TKOF THRUST CALCULATIONS MAY HAVE BEEN JUST A BIT OFF. THE 'ONBOARD PERFORMANCE SYS' SOFTWARE PROVIDES A POP-UP WINDOW WHICH ASKS THE OPERATOR TO CLICK YES OR NO TO THE ACFT NUMBER CURRENTLY SET. IT IS VERY EASY TO BLOW THROUGH THIS BY SIMPLY CLICKING 'YES' AS I DID BECAUSE EACH LAPTOP COMPUTER STAYS WITH ITS PARTICULAR ACFT; CHARGING IN ITS OWN DOCKING STATION IN THE COCKPIT. THERE IS NO REASON TO SUSPECT THAT THE ACFT NUMBER SET WOULD BE ANYTHING DIFFERENT THAN THE ACFT THE COMPUTER IS IN. WE PERFORM THOUSANDS OF THESE CALCULATIONS EACH YEAR; EACH WITH MANY DATA INPUTS AND VARIABLES; AND MUST PERFORM THEM QUICKLY SO AS TO MAINTAIN SCHEDULE. MY RECOMMENDATION WOULD BE TO ALTER THE SOFTWARE SO THAT THE OPERATOR MUST POSITIVELY SELECT THE PARTICULAR ACFT NUMBER WITH EACH CALCULATION AS APPROPRIATE RATHER THAN JUST CLICKING 'YES' TO THE ACFT NUMBER CARRIED THROUGH FROM PREVIOUS USE.