<p>I have a repository of files which are unrelated to each other but are common to multiple projects. Each project might only need a subset of these files. For example:</p>

<pre><code>/myRepo:
    /jquery.js
    /jquery.form.js
    /jquery.ui.js
</code></pre>

<p>Project A requires <code>jquery.js</code> and <code>jquery.form.js</code>, whereas Project B requires <code>jquery.js</code> and <code>jquery.ui.js</code></p>

<p>I could just do a checkout of <code>myRepo</code> into both projects, but that'd add a lot of unnecessary files into both. What I'd like is some sort of way for each Project to only get the files it needs. One way I thought it might be possible is if I put just the required files into each project and then run an <code>svn update</code> on it, but somehow stop SVN from adding new files to each directory. They'd still get the modifications to the existing files, but no unnecessary files would be added.</p>

<p>Is this possible at all?</p>

If I understood your question correctly, you want to share code across projects? If so, look at the svn:externals property.
Externals explained in Subversion Red Book
svn:externals in Windows