There are two basic types of RC helicopters, ones with two main rotors that turn counter to each other, which have no tail rotor, and ones that have a single main rotor and a tail rotor. The tail rotor is there to counteract the torque effect that the main rotor creates. A gyroscopic controller is used to coordinate the main and tail rotors so the helicopter can fly straight. When this system is not working correctly the helicopter will spin in circles.
Test the tail rotor motor and see if it seems strong and responsive. Brushed motors, like those commonly used to drive tail rotors, can wear out in a relatively short time. Replacing the motor may solve your problem if the gyro seems to be working.