I've found a few questions regarding this same problem - IAS service keeps stopping immediately after starting with nothing in the logs to indicate why. We are not running DNS from the same server, although we have added the required registry values to reserve the ports that IAS uses.
I have checked the following KB's and they are not applicable to what I'm seeing:
There is no entry in the application log, and this is not a remote RADIUS server. We are running IAS on windows 2003 server and this problem only began occurring withing the last two weeks.
Things we have changed regarding the server and network recently: We updated the ASA IOS versions for all of our firewalls, then subsequently rolled them back. Don't see how this could cause the service stopping though.
installed PRTG network monitoring tool on the same server, although I am fairly certain the IAS authentication wasn't working by then.
I'm sure you will need more to go on, so ask away.