PWM frequency

I need to output a 100Hz PWM signal but the reference for the analog write says that it is fixed at around 490Hz, is there any way of changing this ? or are there alternative libraries ?

That is based on certain assumptions they made in the library. You can have other frequencies.

http://www.gammon.com.au/forum/?id=11504

so I'd need to use an alternative library or function ?

Take a look at the PWM library

http://arduino.cc/forum/index.php/topic,117425.0.html