Rightclick on the ISO file, go down to PowerISO->Set number of drives. Set it to 1. Rightclick again and choose mount to (driveletter)

If it's a game, then most have no-cd cracks available, because not even even legitimate buyers like the bother of needing the CD-ROM in the drive every time they want to play a game.

Just Google "name of game"+nocd (or no-cd)

BTW Deamon Tools is a good program as well. PowerISO used to have the edge, but now I think they are equal... not that I've tried it, since PowerISO never failed me yet.