<p>I have a small diagnostic VB.Net application ( 2 forms, 20 subs &amp; functions) written using VB.Net 2008 that targets Framework 2.0 and higher, but now I realize I need to support Framework 1.1.   I'm looking for the most efficient way to accomplish this given these constraints:</p>

<ul>
<li>I don't know which parts of the application are 2.0-specific.  </li>
<li>I could reconstruct the forms without too much trouble.   </li>
<li>I need to support SharpZipLib</li>
</ul>

<p>My current idea is to find and install VB.Net 2003, copy over my code and iteratively re-create the tool.  Are there better options?</p>

Probably not. If you don't understand which bits are 2.0-specific, you're probably going to have to go the trial-and-error route. However, you can probably save yourself quite a bit of work if you go looking for generics beforehand. In my experience, those are the most numerous 1.1-incompatible bits that tend to make it into my code.