Can somebody gelp me with this.

I need to hold the weight of two nxt motors which are in an arm with one motor and i need to perform presice movement. So i need ot use holdbrake command..Is it available in robotc..or suggest me another program which can do this task ...Thank you.

I believe the solution for this is two fold; settting the bFloatDuringInactiveMotorPWM to false should prevent the motor from coasting when no power is applied (this is default). The other is gearing and creates a bit of a dilemma; by setting a larger gear on the control motor (can't think of a better way to describe it) you will help with the torque issue and ensure that the motor stays put under the weight of the load, but at the cost of precision (it will speed up the movement of the attachment). Conversely, you can attach a larger gear on the attachement relative to the control motor. This will increase your precision but also increase the likelyhood that the control motor will be moved by the torque of the load.

Thanks JohnWatson for your reply...But i think i have tested the condition bFloatDuringInactiveMotorPWM(motor1) = False ..but it didn't work out quite well..i'll post the code soon as i am not accessible to it now.

On R/C rock crawlers, the use of a worm gear serves this purpose. It provides a large gear reduction for strength, as well as prevents the load from propagating back through to the motor. I haven't tried this myself on LEGOs, but I think it would work well and would be my first attempt if I needed it.