Getting to the Command Prompt
Click on the Start button, then select Programs, then Accessories, then "Command Prompt". In some versions of Windows, the menu selections will have different names, but with luck you will be able to figure it out. A new window will open up, probably with a black background. This is where you type the commands I've given you.
Unfortunately, I don't have a Mac yet, so I have no idea how to do this. But if you need help with this, send an email to firstname.lastname@example.org and I'll find someone who knows Macs to help us.
Without knowing what flavour of Linux you have, it's difficult to provide specific instructions. But somewhere in your menus you probably have something called "console", "terminal", "konsole", "shell prompt", or something like that. If you need more help, send an email to email@example.com, tell me what flavour Linux you have (e.g. Red Hat, Ubuntu, SuSe) and I'll help you figure it out.