Your frustration is understandable! But don't let it get to you too much. This really is a tough one, and I've run into the "cannot set owner" issue on other people's computers myself and was quite perplexed.
There's nothing wrong with mentioning force-delete tools. I'm assuming Unlocker
was one of the two you tried. It works well but does have its limits.
The best solution I can think of for now is to either boot into the Windows Recovery Environment (from your setup DVD or rescue disc, if you have either one) and use the command shell to delete the .iso file.
Another possibility would be to download a free Linux distribution such as Ubuntu, burn it to a CD and then directly boot from the CD into Ubuntu. It should allow you to browse the NTFS partitions on your harddisk and provide another way to simply delete the offending file.
Please let us know if you can boot to the Windows rescue environment, and how comfortable you feel with using the command window. We'll go from there.
We'll get this sorted out.