I did a migration install form SBS 2003 to SBS 2011. I have done a number of these and each one has had a major issue at some point in the process. This one seemed to go perfectly smoothly for once with no errors or false starts until I installed Trend Micro Worry Free Business in order to migrate the accounts. 

Trend installs fine. The messaging component seems to work. However, the console web site does not work. I simply get a 500.19 error and I spent days trying to reinstall, reconfigure permissions on config files and all kinds of stuff.

finally I realized that there is NO NT AUTHORITY\IUSR or NT AUTHORITY\LOCAL SERVICE account in the IIS_IUSRS group. AND I CANNOT ADD THEM. They don't seem to exist on the machine. 

I checked other installs and attempting to re-add IUSR or LOCAL SERVICE on those installs results in a message that the user is already added. On this install, it simply has no idea what those users are. 

The OWA site works fine. The remote workplace site works fine. They seem to use other user accounts for authentication. 

How can this be fixed? Is there a way to force the system to create the users? Or is there  a way to safely reinstall iis without breaking Exchange or other SBS functions. The thing is so tightly knit, I'm afraid to go messing with components. I saw this link on repairing SBS components, but unless I'm missing it, I don't see a way to fix IIS in there. 

I can't unmigrate and start over. Too far along in this process and it's simply not possible at this point.