Alright, that is sketchy. I don't know what the problem is but if I had to guess, you might be failing some pre-reqs. If you've used this same iso to install on a 64bit server and a 32bit server, that leads me to believe the problem is the server on which you are trying to install.

*just thinking*

Even if there were an database instance already on the server (through visual studio or some other application), it should still give you the option to install a second instance.