B737 PERFORMANCE COMPUTER DID NOT ACCOUNT FOR VARIOUS MEL ITEMS.

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ABOUT 1 YR AGO MY AIRLINE SWITCHED TO ANOTHER COMPUTER SYS IN AN EFFORT TO BECOME Y2K COMPLIANT. UNFORTUNATELY OUR AUTOMATED WT AND BAL SUFFERED A GREAT SETBACK AS COMPARED TO THE OLD SYS. IN AN EFFORT TO BECOME MORE AUTOMATED; OUR OPS ENGINEERING DEVELOPED SOME NEW SCREENS TO HELP DISPATCHERS AUTOMATE TKOF AND LNDG PERFORMANCE PENALTIES. DURING THIS EVENT; THE B737-4 HAD ANTI-SKID SYS; AUTO GND SPOILER SYS; AND THE L ENG PMC ALL INOP. THE PENALTIES WERE APPLIED USING THE NEWLY DEVELOPED SCREENS. FORTUNATELY; THE CAPT QUESTIONED THE WT AND BAL NUMBERS AND ASKED TO BE SENT NEW NUMBERS WITHOUT IMPROVED CLB. NEITHER CENTRAL LOAD PLANNING NOR MYSELF THE DISPATCHER; COULD FIGURE OUT HOW THE FLT WAS GIVEN IMPROVED CLB TKOF WTS WHEN WE DID NOT INITIATE THOSE NUMBERS. FURTHERMORE; WE ARE NOT AUTH TO USE IMPROVED CLB WITH ANTI-SKID INOP. IT TURNED OUT THAT THE AUTOMATED SCREEN USED TO APPLY TKOF WT PENALTIES HAD USED IMPROVED CLB WTS WHICH GAVE AN ERRONEOUS MAX TKOF WT THAT WAS 7000-8000 LBS TOO HIGH. THE FLT RETURNED TO GATE; BUT IF IT WERE NOT FOR THE CAPT THAT KNEW BY LOOKING AT HIS V-SPDS THAT THIS WAS AN IMPROVED CLB TKOF; THE FLT WOULD HAVE DEPARTED 4000 LBS OVERWT. IT IS MY UNDERSTANDING THAT FLCS ARE NOT SPECIFICALLY SHOWN ON THEIR ACARS THAT THE WT AND BAL NUMBERS ARE BASED ON IMPROVED CLB OR NOT. THIS ACFT ALONG WITH THE SAME MEL'S HAD BEEN FLYING THE LINE PREVIOUSLY FOR 3-4 DAYS. MOST LIKELY THE PREVIOUS FLTS WERE ALL ISSUED IMPROVED CLB TKOFS WHEN THIS WAS NOT AUTH AND FLCS ALONG WITH DISPATCHERS AND LOAD AGENTS WERE NOT AWARE OF THIS.