title: c# app doesn't work on XP
body: <p>i made a app that uses the <code>watIn library</code>, and target to .NET 3.5 (so it will work on XP to).
  after i published the app on my PC, i installed it, and it worked well. but, when i tried it on different PC running XP, it didn't work. i made sure the PC I'm testing on is running .NET 3.5 or grater.</p>
  
  <p>I'm running windows 7 and visual studio 2010.</p>
  
  <p>the program work on Windows 7 (tried that on 2 PC's).
  the program doesn't work on XP (tried that on 2 PC's).</p>
  
  <p>On Xp i get this message:</p>
  
  <pre><code>Unable to install or run the application. The application requires that
  assembly WindowsBase Version 3.0.0.0 be installed in the Global
  Assembly Cache (GAC) first.
  
  Please contact your administrator 
  </code></pre>
  
  <p>my questions:</p>
  
  <ol>
  <li><p>what should i do to make sure it work on XP?</p></li>
  <li><p>if i start the development on .net 4, and then switch it to 3.5 and it still run, that mean i only used 3.5 code/features from the get go? </p></li>
  <li><p>will visual studio give me compile Error if i run 4.0 code on app that i change his properties to target .net 3.5 in the middle of development?</p></li>
  <li><p>is there a way to install the .net 3.5 framework with my app installer? (if it doesn't already got 1 on the PC)</p></li>
  </ol>
  
  <p>(sorry for my English)</p>