Visual Studio Won't Start

Visual Studio 2013 started hanging on startup, a CPU core would be maxed out but there would be no events in Process Monitor.

I'm sure the same debugging rules apply to 2010 & 2012...

1. It will probably run in safe mode: devenv.exe /safemode
2. Might be worth a bet resetting settings: devenv.exe /resetsettings
3. Or viewing a log for warnings: devenv.exe /resetsettings
   Check out the log file in:
   C:\Users\_USERNAME_\AppData\Roaming\Microsoft\VisualStudio\_VERSION_
4. As a last resort (takes ages) try repair by Control Panel > Add/ Remove Programs