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I've never used IIS for Xoops, or any hosting package for that matter.
If you a run a search on ISS Permissions, you might be able to gain information from others experience, but I suspect you did this already.
All I can suggest is that check for other folders/files that may need special permission via IIS. It has been done before, but the answers are harder to find then your common chmod related questions.
I'm no IIS expert, so bear with me...
Check for access rights on common.php
Make sure that you also give the IUSR account read/write/modify permissions to any modules that have a cache directory or you'll run into more issues....
Set permissions for /template_c directory and cache directories
Check that your mainfile.php is writeable by all during install, then set back to something tighter for just yourself
I'm sure there are others that I am missing, and I sure others will be able to contribute. There are quite a few people that use IIS here.