B737 FLT CREW REPORTS FMC PROGRAMMING DIFFICULTIES DURING PREFLIGHT; AND A COURSE DEVIATION DURING THE INITIAL SEGMENT OF THE COWBY 2 RNAV DEPARTURE FROM LAS.

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DURING FMC PREFLT PROGRAMMING; WE NOTICED AN ALT CONSTRAINT AT ROPPR MESSAGE WHICH WOULD NOT ALLOW US TO ACTIVATE RTE. WE DELETED ALT CONSTRAINT AT ROPPR AND ACTIVATED RTE AND THEN PUT THE ALT RESTR BACK IN AT ROPPR AND ALL SEEMED FINE. WE DID VERIFY THAT THE CRUISE ALT WAS SHOWING FL350 AND NOT 3500 FT. WE VERIFIED THE CORRECT DEP WITH THE FIRST FIX OF RBELL AND THE SECOND FIX OF ROPPR; ETC. PRIOR TO TAKING RWY 25R FOR TKOF; THE FO UPDATED THE RWY AND WE MADE SURE NAV SWITCHES WERE DOWN. AT 400 FT ON DEP; THE FO SELECTED LNAV AND WE IMMEDIATELY NOTICED A CDI DEFLECTION TO THE L OF 1 DOT. THE FO WAS TRYING TO CORRECT THE CDI DEFLECTION AND FLYING A HDG OF APPROX 230 DEGS AND HE ASKED ME IF THIS LOOKED RIGHT. I STATED NO BECAUSE WE SHOULD BE ON APPROX A 255 DEG HDG. AT THAT TIME; LAS TWR ASKED IF WE WERE OK BECAUSE WE WERE 1 MI L OF COURSE AND I TOLD HIM WE WOULD BE UNABLE TO CONTINUE ON THE RNAV DEP. HE GAVE US A HDG TOWARDS ROPPR AND I ASKED HIM IF THIS ROPPR WAS 12 O'CLOCK POS AND 5 MI AS IT APPEARED ON THE LEGS PAGE. HE GOT THE EXACT DISTANCE AND IT APPEARED WE WERE SHOWING ABOUT A 1 MI DEV. I DO NOT BELIEVE THAT THERE WAS ANY TFC CONFLICT AND LAS TWR MADE NO REF TO ANY PROB CAUSED BY THE DEV. DEP CONTINUED VECTORING US TOWARDS COWBY AND IT APPEARED THAT BY THAT TIME THE FMC HAD UPDATED ITSELF AND WAS GIVING ACCURATE DATA. THE REST OF THE FLT WAS UNEVENTFUL. WE DON'T KNOW WHAT CAUSED THE DEV AND WITHOUT A MOVING MAP TO DEPICT EXACT LAYOUT OF THE RTE; IT IS EXTREMELY HARD TO TELL WHAT CAUSED THIS ERROR.