Ok after further diggin' i finally bumped into a solution. Turns out IIS7 had url redirection rules enabled, so this can be accomplished through web.config, like this
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="Redirect to WWW" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTP_HOST}" pattern="^yoursite.com$" />
</conditions>
<action type="Redirect" url="http://www.yoursite.com/{R:0}" redirectType="Permanent" />
</rule>
</rules>
</rewrite>
I overlooked it before cause it was listed as ASP.NET solution, not classic ASP. But there it is, solved.