(And does anyone know of a good website for experienced Windows users who are new to Macs?) But I'm really struggling to work out how to use a Mac.

I would really like to be a Mac bigot - I'm sure it's my calling. How would I do this? I've looked at the menu options in Disk Utility and see nothing useful. I've done a lot of searching, and I've found a reference to 'repairing associations'. But now, when I click on (any) DMG file, all that happens is that 'Disk Utility' opens. I KNOW that I've opened these before, because I can see some on the desktop - eg Firefox, which is installed and working. Right now, I have been trying to install which is delivered via a DMG file. I have a Mac (a PowerBook G3) which I got second hand, in order to find out a bit about the Apple world (and to give my wife a machine she can use for her emailing). I'm a very experienced Windows user, and I'd have thought I could manage this. It's so horribly embarrassing to have to ask this.