Make sure to add and set local computer IUSR built-in account to read write accesss only to the folders that need it.

If you are getting a 404 error when trying to access /admin allowoverride is handled by URL rewrite.

https://github.com/getgrav/grav/issues/915

In IIS under URL Rewrite. Import the .htaccess file found in root of Grav folder. 8 rules should be created.

https://serverfault.com/questions/275814/how-do-i-convert-this-mod-rewite-rule-to-iis7-urlrewrite