[IIS] How to install ASP.Net 3.5 onto IIS 6 ?

You don't need to.

ASP.Net 3.5 is basically an extension of ASP.Net 2.0, so just register ASP.Net 2.0 to IIS 6 and you are ready to run your ASP.Net 3.5 application on IIS 6.

But there do have some configuration settings you might need to know if you are running you applicaion on web server without VS2008 installed.

If you create your site with VS2008. then these settings are created by VS, but you may need to add the following section to your web.config to make your site compiled with ASP.Net 3.5 if you are modifying an existing site.

<system.codedom> 
   <compilers><compiler language="c#;cs;csharp" extension=".cs" compilerOptions="/warnaserror-" warningLevel="4" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"> 
      <providerOption name="CompilerVersion" value="v3.5"/></compiler> 
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" compilerOptions="/optioninfer+" type="Microsoft.VisualBasic.VBCodeProvider, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"><providerOption name="CompilerVersion" value="v3.5"/> 
   </compiler></compilers> 
</system.codedom> 

Also, your assembly settings should be changed.

<compilation debug="true">
   <assemblies>
    <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
    <add assembly="System.Data.DataSetExtensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
    <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
    <add assembly="System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
   </assemblies>
  </compilation>