what browser are you using?
Are you clicking the search button or using the enter key?
in the main.dwt.php template, try changing the search form code:
<form id="searchForm" method="get" action="../Products_Results.php" >
<input name="Search" type="submit" value="Search" id="Search" alt="Search" />
<input type="text" name="S_ProductName" id="S_ProductName" />
</form>
to:
<form id="searchForm" method="get" action="../Products_Results.php" >
<input name="SearchSubmit" type="submit" value="Search" id="SearchSubmit" alt="SearchSubmit" />
<input name="Search" type="hidden" value="Search" id="Search" alt="Search" />
<input type="text" name="S_ProductName" id="S_ProductName" />
</form>