Of course as a base setup you need PHP and MySQL running. A great source that describes how to achieve this would be the article Using FastCGI to Host PHP Applications on IIS 7.0 over at iis.net. The same is true for MySQL, so check out Setting up MySQL for PHP applications.
In order to install Symphony CMS with full functionality make sure you have the following PHP extensions enabled:

• php_mbstring.dll
• php_mcrypt.dll
• php_mysql.dll
• php_xsl.dll
• php_zip.dll

Before you can start the installation process of Symphony, you have to alter 2 files. Please locate the index.php file in the root folder of the Symphony website. There you should see the lines

define('DOCROOT', rtrim(dirname(dirname(__FILE__)), '/'));
define('DOMAIN', rtrim(rtrim($_SERVER['HTTP_HOST'], '/') . dirname(dirname($_SERVER['PHP_SELF'])), '/'));

right at the beginning.
Change those to

define('DOCROOT', rtrim(dirname(dirname(__FILE__)), '/\\'));
define('DOMAIN', rtrim(rtrim($_SERVER['HTTP_HOST'], '/\\') . dirname(dirname($_SERVER['PHP_SELF'])), '/\\'));

(notice the added double backslashes) and apply the same procedure to the index.php located inside the /symphony folder. As a result you should get:

define('DOCROOT', rtrim(dirname(__FILE__), '/\\'));
define('DOMAIN', rtrim(rtrim($_SERVER['HTTP_HOST'], '/\\') . dirname($_SERVER['PHP_SELF']), '/\\'));

After this step you should be able to install Symphony CMS as you’re used to! The installation process will work now, but there is one more thing you have to adjust. Symphony CMS utilizes the mod_rewrite engine if installed on an Apache web server. The IIS 7.0 pendant to that is called URL Rewrite Module. You’re gonna have to download and install this to get Symphony running. With the Rewrite Module installed you finally need to set up the rules equally to the mod_rewrite rules defined in the .htaccess files. The relevant portion of the web.config file in the root of your website should look like this:

<rewrite>
<rules>
<rule name="Favicon Rule Stopper" stopProcessing="true">
<match url="favicon.ico$" />
<conditions logicalGrouping="MatchAll" />
</rule>
<rule name="Image Rule" stopProcessing="true">
<match url="^image\/(.+\.(jpg|gif|jpeg|png|bmp))$" />
<conditions logicalGrouping="MatchAll" />
<action type="Rewrite" url="/extensions/jit_image_manipulation/lib/image.php?param={R:1}" appendQueryString="false" />
</rule>
<rule name="Add Trailing Slash" stopProcessing="true">
<match url="^(.*)$" ignoreCase="false" />
<conditions logicalGrouping="MatchAll">
<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
<add input="{URL}" negate="true" pattern="/$" ignoreCase="false" />
<add input="{URL}" negate="true" pattern="(.*)/$" ignoreCase="false" />
</conditions>
<action type="Redirect" url="/{R:1}/" redirectType="Permanent" />
</rule>
<rule name="Main Rewrite" stopProcessing="true">
<match url="^(.*)\/$" ignoreCase="false" />
<conditions logicalGrouping="MatchAll">
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
</conditions>
<action type="Rewrite" url="/index.php?page={R:1}&amp;{QUERY_STRING}" appendQueryString="false" />
</rule>
</rules>
</rewrite>

The folder /symphony needs separate rewrite rules. Set up a new web.config file there and set the rewrite-section to

<rewrite>
<rules>
<remove name="Add Trailing Slash" />
<remove name="Main Rewrite" />
<remove name="Image Rule" />
<rule name="Main Rewrite" stopProcessing="true">
<match url="^(.*)\/$" ignoreCase="false" />
<conditions logicalGrouping="MatchAll">
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
</conditions>
<action type="Rewrite" url="/symphony/index.php?page={R:1}&amp;{QUERY_STRING}" appendQueryString="false" />
</rule>
<rule name="Add Trailing Slash" stopProcessing="true">
<match url="^(.*)$" ignoreCase="false" />
<conditions logicalGrouping="MatchAll">
<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
<add input="{URL}" negate="true" pattern="/symphony$" ignoreCase="false" />
<add input="{URL}" negate="true" pattern="(.*)/$" ignoreCase="false" />
</conditions>
<action type="Redirect" url="/symphony/{R:1}/" redirectType="Permanent" />
</rule>
</rules>
</rewrite>

So this really was the last step and your installation of Symphony CMS on IIS 7.0 should be up and running!

In this case, I used the latest release of Symphony, which has the version number 2.0.2. I hope I could help some of you out there.