Thanks Neil, that pointed me in the right direction. I wasn't able to find the registry key at HKLM\Software\ESRI\Desktop10.x., instead it was located at HKLM\SOFTWARE\Wow6432Node\ESRI\ArcGIS - much appreciated.
Correct, on a 64-bit OS, the keys you need will be under the Wow6432Node node when you look at it using RegEdit. However, when you write your code to access the registry you should leave Wow6432Node out of the path. The OS will know to look under that node automatically.