Why is my PC build not running when I convert from Web?

I have a project that I've been working on for a while, and I've been developing for Web deployment. No problems, everything working fine.

When I switch over and try to do a build for PC, everything seems to work fine, but when I run the project, I get the Unity splash icon, fade to black, and complete lockup. I have to kill the process to get rid of it, and the screen never comes back from black. Task Manager reports that the process is 'Not Responding'

I've always been able to switch back and forth between builds with no problems, and as far as I know, I'm not doing anything strange...

Anyone have any ideas?