<xsl:variable> is not supported by the IE5 implementation of XSL.

what you will have to do is something like:

<xsl:template match="rs:data">
     <TABLE>
          <th><INPUT TYPE="BUTTON">
			<xsl:attribute name="sortfld">
				<xsl:value-of select="@TradeDate"/>
			</xsl:attribute>
			<xsl:attribute name="value">
				<xsl:value-of select="."/>
			</xsl:attribute>
		  </INPUT>
	</th>
    </TABLE>
</xsl:template>


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list