A common mistake new Mac users make is just dropping the .ISO image into the burn folder.  I want you to say it with me, WRONG!  NO!  Bad new Mac user!  What you are going to want is a program to burn.

Now, the program I recommend is called Burn.  It does what the name implies,  it burns discs.  Download and install it is as simple as most Mac programs.  The first time it opens,  it will be on the data tab of the program.  What you want to do to burn your .ISO image is switch to the copy tab.  Then locate your .ISO image [Kubuntu for example.]  Drag and drop it in, hit  burn, configure how fast you want it to burn, and hit burn again.  In a matter of 10 minutes or so,  you will have a fresh copy of Kubuntu to throw on your old Windows Machine.