PROBLEM WITH XP-SP2 RE-ACTIVITION

I BOUGH A WINDOWS XP SERVICE PACK 2 AND USED ABOUT 8 MONTHS.
I HAD FORMATED MY COMPUTER FEW TIMES AND ABLE TO ACTIVATE IT ON LINE.
BUT TODAY, WHEN I REFORMAT AGAIN AND THE ACTIVATION HAS DENIED BECAUSE THEY SAID I ACCED THE NUMBER OF ALLOWABLE ACTIVATION.
I THOUGH ORIGINAL OS GOT UNLIMITED USE/REINSTALLATION ON WINDOWS ON THE SAME PC.
ANYBODY MASTER PLEASE HELP! TOLONG! TOLONG!