The quickest solution I can think of with this script provided by Google.
Basically the searching is done through Google, and results will only be of what Google has found on your web site. However, it is quick and painless.
<!-- SiteSearch Google -->
<FORM method=GET action="http://www.google.com/search">
<input type=hidden name=ie value=UTF-8>
<input type=hidden name=oe value=UTF-8>
<TABLE bgcolor="#FFFFFF"><tr><td>
<A HREF="http://www.google.com/">
<IMG SRC="http://www.google.com/logos/Logo_40wht.gif"
border="0" ALT="Google"></A>
</td>
<td>
<INPUT TYPE=text name=q size=31 maxlength=255 value="">
<INPUT type=submit name=btnG VALUE="Google Search">
<font size=-1>
<input type=hidden name=domains value="YOUR DOMAIN NAME"><br><input type=radio name=sitesearch value=""> WWW <input type=radio name=sitesearch value="YOUR DOMAIN NAME" checked> YOUR DOMAIN NAME <br>
</font>
</td></tr></TABLE>
</FORM>
<!-- SiteSearch Google -->
Just make sure you put your domain name where it says YOUR DOMAIN NAME.
Sorry that its not javascript. Hope you weren’t looking just for a javascript solution. If you were, my solution wouldn’t work.