By default WSS 3.0 runs under the .NET 2.0 framework. To change this to use the .NET 3.5 framework execute the following steps:

1. Open the web.config for your site
2. Add the following assemblies below the tag:

   <add assembly="Microsoft.SharePoint, Version=12.0.0.0, 
   	Culture=neutral, PublicKeyToken=71e9bce111e9429c" /> 
   <add assembly="System.Core, Version=3.5.0.0, 
   	Culture=neutral, PublicKeyToken=B77A5C561934E089" /> 
   <add assembly="System.Xml.Linq, Version=3.5.0.0, 
   	Culture=neutral, PublicKeyToken=B77A5C561934E089" /> 
   

3. Then add the following to right before (for example) the tag

   <system.codedom> 
     <compilers> 
       <compiler language="c#;cs;csharp" extension=".cs" warningLevel="4" 
         type="Microsoft.CSharp.CSharpCodeProvider, System, Version=2.0.0.0, 
         Culture=neutral, PublicKeyToken=b77a5c561934e089"> 
         <providerOption name="CompilerVersion" value="v3.5" /> 
         <providerOption name="WarnAsError" value="false" /> 
       </compiler> 
     </compilers> 
   </system.codedom>
   

4. Your site now runs under the .NET Framework 3.5!