><xsl:for-each select="doc">
>         <xsl:variable name="name"><xsl:value-of select="@id"
/></xsl:variable>
>         <xsl:apply-templates select="document('C:\{$name}.xml')" />
></xsl:for-each>

try
<xsl:variable name="docconcat" select="concat('c:\',$name,'.xml')"/>
<xsl:apply-templates select="document($docconcat)"/>


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