You can use the <xsl:attribute> element and apply it directly to (inside) the <xsl:copy> element:
<xsl:template match="Component">
<xsl:copy>
<xsl:attribute name="Win64">yes</xsl:attribute>
<xsl:apply-templates select="node()|@*"/>
</xsl:copy>
</xsl:template>
Update: Embedded in an XSLT that otherwise copies the Xml contents, this looks like this:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="node()|@*">
<xsl:copy>
<xsl:apply-templates select="node()|@*"/>
</xsl:copy>
</xsl:template>
<xsl:template match="Component">
<xsl:copy>
<xsl:attribute name="Win64">yes</xsl:attribute>
<xsl:apply-templates select="node()|@*"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
Update 2: This assumes you only want to add Win64="yes" to <Component> elements. If this is not the case, you will have to adapt the XPath expression of the match attribute in the template that inserts your additional attribute.
Update 3: Well-formed input and output documents:
I assume this as the input document:
<?xml version="1.0" encoding="UTF-8"?>
<xml>
<Component Id="cmp25217AE65B163B199EDDA7F29198730A" Guid="{DEB29383-8BF1-4FD0-830B-DF8639F4069A}"/>
<Component Id="cmp93E1B1FFA5A62A43251E23BD65FBAA66" Guid="{76E8B8CE-835D-498E-9330-CE940C9510BF}"/>
<Component Id="cmp3D7B898C57056B0E87C3A964112BB9D6" Guid="{3BA9A892-C44F-4B2E-B0B9-B732120D35DB}"/>
</xml>
Then, the output of the aforementioned XSLT looks like this:
<?xml version="1.0" encoding="utf-8"?>
<xml>
<Component Win64="yes" Id="cmp25217AE65B163B199EDDA7F29198730A" Guid="{DEB29383-8BF1-4FD0-830B-DF8639F4069A}" />
<Component Win64="yes" Id="cmp93E1B1FFA5A62A43251E23BD65FBAA66" Guid="{76E8B8CE-835D-498E-9330-CE940C9510BF}" />
<Component Win64="yes" Id="cmp3D7B898C57056B0E87C3A964112BB9D6" Guid="{3BA9A892-C44F-4B2E-B0B9-B732120D35DB}" />
</xml>