If you're having a problem with Visual Studio 2015 not launching then, perhaps, you're in the same boat as me. I just installed VS 2015 CTP and when I went to launch it the splash screen would blink up then disapear at once. Staring with SafeMode didn't help and there was nothing in any log I could find to explain it. In the end I found the solution was to open up regedit and delete the 14.0 directories under HKEY_CURRENT_USER\Software\Microsoft\VisualStudio. Any settings you had will disapear but it isn't like you could get into Visual Studio to use those settings anyway.
Hopefully this helps somebody.