For more information about what PWM is see:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html
For information about stepping motors see:-
http://www.thebox.myzen.co.uk/Workshop/Motors_3.html
and
http://www.thebox.myzen.co.uk/Workshop/Motors_4.html
however when I get to 30 minutes I start to get unexpected results.
It also helps to know what results these are and what you expect.