Today I got the wild idea that disabling autorun would increase the security of my system. I set about trying to find out how to do it and what the settings are. I found an MSDN article at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/Shell/programmersguide/shell_basics/shell_basics_extending/autorun/autoplay_reg.asp that seemed to be exactly what I am looking for. It lists 3 ways to disable autorun. The method that looked the best to me is to modify the registry. The documentation gives the following bit numbers and what they disable.

When I went into the registry I found a value of 0x91. I am wondering what setting the lowest order bit does? A check of a couple of other systems reveals the same setting so it doesn't appear to be a random number, just inadequate documentation.