This appears to be correct.
You could also use something like:
open=command /c start /max index.htm
You might want to try that just to see if it works.
However, I suspect the simplest answer would be that you have autoplay disabled - perhaps though some previous tweaking of Windows? Or via a Group Policy if it is a PC on a Windows Domain. If you insert a blank CD-R or DVD-R, do you get the pop-up dialog? If not, then autoplay is disabled. You need autoplay enabled in order to run autorun.inf
If autoply is disabled and if you are confident editing the registry and you are not on a PC belonging to your enterprise (unless you have authority), you could try editing the registry. I believe the correct entry is found at:
There should be an entry called "AutoRun" and it should be set to 1 not 0.
To check the group policies, run gpedit.msc and go to the section "Windows Components \ AutoPlay Policies". The set "Turn off Autoplay" to disable.
Microsoft provide some pointers to autoplay security issues here.