Pretty Permalinks Issue on Windows Server

Hello.
I have a WP site running under a subdomain to my main site. Everything is working well except the Pretty Permalinks. I can post all day long using the default ?p=123 style URL, but as soon as I turn on PrettyLinks, everything bombs. I found a rule for web.config that I would need to put in place for these to work, but I need this to only effect my pages under the subdomain (which is subdirectory under the main site).

Here is the rule:

<rule name="PrettyLinks Rule" stopProcessing="true">
            		<match url=".*" />
            		<conditions logicalGrouping="MatchAll">
               		 	<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
               		 	<add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
            		</conditions>
            		<action type="Rewrite" url="index.php" />
       		</rule>	

What needs to change on this rule to get this to work correctly without effecting my main site? Or this just completely wrong?

Thanks in advance for reading.

Chris.