Once you have downloaded and installed ScummVM, open it up (Start button, Programs, ScummVM). Make sure the Discworld CD is in the PC. Select 'Options' button on the ScummVM program, select 'Graphics' and ensure 'full screen' is ticked - press Ok. Now, in ScummVM press 'Start' (top right) and keep hitting 'Go Up' button until you see a list of your drives (C:, D:, E: etc) select the one with the Discworld CD in and press OK and finally the 'Start' button again in the top right.
Now the game will start and everytime you load ScummVM it will remember where the iscworld game is and be ready to play.
Let me know if this helps or you need more info...