Its actually relatively straight forward to get working. If you are able to see the web admin interface then you have installed the self-contained version of GeoServer which as Imp points out runs using the Jetty servlet container.
The trick to exposing GeoServer through IIS 7 lies in the use of Application Request Routing (ARR). With ARR it is possible to redirect traffic coming in to IIS based on rules, one such rule type uses a regular expression pattern to match incoming URL requests.
So, using ARR it is possible to redirect traffic to GeoServer running on port 8080 based on some public url. For example, if you have a public url of http://www.myserver.com/geoserver then you can use ARR to get IIS to pass all requests matching that url to localhost:8080 (assuming your GeoServer is running on the same server as IIS 7).
The following post on the IIS Admin blog has a good example of using ARR to route requests to Tomcat. Skip page 1 as that discusses how to install Tomcat which is irrelevant for you since you already have GeoServer running on port 8080. The remaining details on pages 2 and 3 will get you up and running with ARR. All you will need to do is map incoming requests to localhost:8080.
This is how I have my Windows 2008 servers configured, although I am using Tomcat and the GeoServer web archive rather than the Windows GeoServer installer, but the principle is the same.
Hope that helps!