>Please try to change your test condition to the following:
>
>

><xsl:choose>
>	<xsl:when test="string(number(.))!='NaN'">
>		<Data Type="Number">
>			<xsl:value-of select="."/>
>		</Data>	
>	</xsl:when>
>	<xsl:otherwise>
>		<Data Type="String">
>			<xsl:value-of select="."/>
>		</Data>
>	</xsl:otherwise>
></xsl:choose>
>

>
>Notice that I have also exchanged the "if" construction to "choose" but this has no influence on the solution. I did that only to keep the conditional logic in one place.

This is the Xls as I have it now but I cannot process it with that change:

<xsl:stylesheet version="1.0"
    xmlns="urn:schemas-microsoft-com:office:spreadsheet"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
 xmlns:msxsl="urn:schemas-microsoft-com:xslt"
 xmlns:user="urn:my-scripts"
 xmlns:o="urn:schemas-microsoft-com:office:office"
 xmlns:x="urn:schemas-microsoft-com:office:excel"
 xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet" > 
 
<xsl:template match="/">
  <Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet"
    xmlns:o="urn:schemas-microsoft-com:office:office"
    xmlns:x="urn:schemas-microsoft-com:office:excel"
    xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet"
    xmlns:html="http://www.w3.org/TR/REC-html40">
    <xsl:apply-templates/>
  </Workbook>
</xsl:template>


<xsl:template match="/*">
  <Worksheet>
  <xsl:attribute name="ss:Name">
  <xsl:value-of select="local-name(/*/*)"/>
  </xsl:attribute>
    <Table x:FullColumns="1" x:FullRows="1">
      <Row>
        <xsl:for-each select="*[position() = 1]/*">
          <Cell><Data ss:Type="String">
          <xsl:value-of select="local-name()"/>
          </Data></Cell>
        </xsl:for-each>
      </Row>
      <xsl:apply-templates/>
    </Table>
  </Worksheet>
</xsl:template>


<xsl:template match="/*/*">
  <Row>
    <xsl:apply-templates/>
  </Row>
</xsl:template>


<xsl:template match="/*/*/*">
  <Cell>
	<xsl:when test="string(number(.))!='NaN'">
		<Data Type="Number">
			<xsl:value-of select="."/>
		</Data>	
	</xsl:when>
	<xsl:otherwise>
		<Data Type="String">
			<xsl:value-of select="."/>
		</Data>
	</xsl:otherwise>
</Cell>
</xsl:template>


</xsl:stylesheet>

I also tried to insert the choose tag after the cell tag and the closing tag before the Cell closing tag but this doesn't work either.