Just create a web.config  in wordpress root dir:

<?xml version="1.0"?>

<configuration>

    <system.webServer>
        <rewrite>
            <rules>
                <rule name="Main 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>
            </rules>
        </rewrite>
     </system.webServer>
</configuration>

Read more:

WordPress Pretty Permalinks on IIS 7.0

http://codex.wordpress.org/Using_Permalinks