Well, this looks like it would be best done client-side, but if you need a server-side solution, you want to do something like:

<xsl:if test="umbraco.library:RequestForm('nationalClient') = 'allClients'">
<!-- Display this list when "All clients" is selected -->
...
</xsl:if>
<xsl:if test="umbraco.library:RequestForm('nationalClient') = 'nationalClient'">

<!-- Display this list when "National clients" is selected -->
...
</xsl:if>

You can use RequestQueryString if your form is going to be a GET form.

Actually, thinking about it, you will want to deal with no postbacks as well, so you actually want:

<xsl:choose>
    <xsl:when test="umbraco.library:RequestForm('nationalClient') = 'nationalClient'">
        <!-- Display this list when "National clients" is selected -->
        ...
    </xsl:when>
    <xsl:otherwise>
        <!-- Display this list when "All clients" is selected, or no postback -->
        ...
    </xsl:otherwise>
</xsl:choose>

Thanks Rob I thought it would involve a "choose" tag.

I still need help with the select menu though, and the form. Can anyone help with this?

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet [ <!ENTITY nbsp "&#x00A0;"> ]>
<xsl:stylesheet 
  version="1.0" 
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
  xmlns:msxml="urn:schemas-microsoft-com:xslt" 
  xmlns:umbraco.library="urn:umbraco.library" xmlns:Exslt.ExsltCommon="urn:Exslt.ExsltCommon" xmlns:Exslt.ExsltDatesAndTimes="urn:Exslt.ExsltDatesAndTimes" xmlns:Exslt.ExsltMath="urn:Exslt.ExsltMath" xmlns:Exslt.ExsltRegularExpressions="urn:Exslt.ExsltRegularExpressions" xmlns:Exslt.ExsltStrings="urn:Exslt.ExsltStrings" xmlns:Exslt.ExsltSets="urn:Exslt.ExsltSets" xmlns:uTube.XSLT="urn:uTube.XSLT" xmlns:Designit.VideoEmbed="urn:Designit.VideoEmbed" xmlns:PS.XSLTsearch="urn:PS.XSLTsearch" 
  exclude-result-prefixes="msxml umbraco.library Exslt.ExsltCommon Exslt.ExsltDatesAndTimes Exslt.ExsltMath Exslt.ExsltRegularExpressions Exslt.ExsltStrings Exslt.ExsltSets uTube.XSLT Designit.VideoEmbed PS.XSLTsearch ">

<xsl:output method="xml" omit-xml-declaration="yes" />

<xsl:param name="currentPage"/>

<xsl:variable name="documentTypeAlias" select="string('ProjectPage')"/>
<xsl:template match="/">

<!-- The fun starts here -->
    <form action="#">
  <div id="news_filter">
    <fieldset>
        <select name="nationalClient" id="newsCategory">
          <option value="allClients">All clients</option>
          <option value="nationalClient">National clients</option>
        </select>
      </fieldset>
  </form>
    </div>  
  <div id="clients_list_container">
    <ul>
      <xsl:choose>
        <xsl:when test="umbraco.library:RequestForm('nationalClient') = 'nationalClient'">
          <!-- Display this list when "National clients" is selected -->
          <xsl:for-each select="$currentPage/ancestor-or-self::root//* [name() = $documentTypeAlias and string(umbracoNaviHide) != '1']">
            <xsl:sort select="@nodeName" order="ascending"/>
            <xsl:if test="string(nationalClient) = '1'">
              <li>
                <a href="{umbraco.library:NiceUrl(@id)}" title="{@nodeName}">
                  <img src="{concat(substring-before(clientLogo,'.'), '_thumb_120.jpg')}"/>
                </a>
              </li>
            </xsl:if>
          </xsl:for-each>
        </xsl:when>
        <xsl:otherwise>
          <!-- Display this list when "All clients" is selected -->
          <xsl:for-each select="$currentPage/ancestor-or-self::root//* [name() = $documentTypeAlias and string(umbracoNaviHide) != '1']">
            <xsl:sort select="@nodeName" order="ascending"/>
            <xsl:if test="string(displayInClientsList) = '1'">
              <li>
                <a href="{umbraco.library:NiceUrl(@id)}" title="{@nodeName}">
                  <img src="{concat(substring-before(clientLogo,'.'), '_thumb_120.jpg')}"/>
                </a>
              </li>
            </xsl:if>
          </xsl:for-each>
        </xsl:otherwise>
      </xsl:choose>
    </ul>
  </div>

</xsl:template>

    
</xsl:stylesheet>

What help do you need? :o)

It seems like you're building a form where:

1. user selects either All Clients or National Clients
2. the form posts back to the page with the appropriate list

That aside, I'm not sure "#" is a valid form action? You might need to tell it explicitly to post back to the current page.

Also, is there a reason you've named the form field the same as the option value? (nationalClient)

If you leave the form action blank then it will post back to the current page URL; I would assume that the hash will maintain that behaviour as it shouldn't affect server behaviour at all.

As shown above a server side solution is missing one major thing and one possible thing:

1. You have no submit button, so unless you have a change event handler set up in other code that submits the form you'll never get a postback.

2. I seem to recall that the default method for forms if no specified is actually GET, so you'll eithre need to change my example to use umbraco.library:RequestQueryString or you'll have to add method="post" to your form tag.

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