BFR THE FLT, THE ARPT OWNER ADZD THE PLT NOT TO FLY, SINCE HE (THE PLT) APPEARED TO BE INTOXICATED. THE PLT 'SAID OK & LEFT,' BUT RETURNED APRX 1 HR LATER. A WITNESS RPRTD THE PLT ATMTD TO TAKEOFF WITH FULL FLAPS. DRG THE TAKEOFF ROLL, THEACFT DRIFTED OFF THE RWY, HIT SOME PLASTIC MARKERS, THEN REENTERED THE RWY & CONTD TAKING OFF. ACCORDING TO 2 WITNESSES,THE PLANE WAS IN A NOSE HI ATTITUDE WITH LOW SPEED AFTER LIFT-OFF & CONTD FLYING JUST ABOVE THE GROUND. SUBSEQUENTLY, ITENTERED A LEFT BANK & CRASHED INTO TREES NEAR THE END OF THE RWY. AFTER THE ACDNT, THE FLAPS WERE FND FULLY EXTDD; THE PLT'S SEAT BELT WASN'T FASTENED & HE WAS SERIOUSLY INJURED. THE RGT FNT (PAX'S) SEAT WAS FND OFF THE SEAT TRACK; THE FNTSEAT STOPS FOR THAT SEAT HAD NOT BEEN INSTALLED. NO OTHER MECHANICAL PRBLM WAS FND. THE PLT STATED THAT HIS SEAT SLIPPEDBACKWARD ON TAKEOFF & THE PAX GRABBED THE CTL WHEEL & PULLED BACK, CAUSING THE ACFT TO STALL. POH RCMDD A MAX OF 10 DEG FLAPS FOR TAKEOFF. A BLOOD SAMPLE TAKEN JUST AFTER THE ACDNT SHOWED THE PLT HAD AN ALCOHOL LVL OF 183 MG/DL (0.183%).