After my computer required a restart this morning, I had the same problem with TMG8 refusing to open. Nothing happened when I clicked the icon or used the Start menu. I ran the Microsoft FixIt program and checked the registry for the values shown at the Microsoft website. TMG would open once after each restart, but as soon as I did some typing/editing, TMG would just disappear.
I also tried turning off Norton SONAR and setting TMG to open with the welcome window. Each time, TMG would start once after the restart and suddenly close after I typed a few things.
My computer was set up to open in Normal mode - which Chappy's wasn't. So I changed to Basic mode, and voila, TMG seemed to be working OK. I decided to change back to Normal startup mode and restart my computer again. TMG opened and allowed me to edit a tag. I haven't done much yet, but I think it's still working.
Seems like a strange thing to happen - TMG has always worked flawlessly for me before. Does anyone know what's going on?