<Grid.DataContext>
  <XmlDataProvider Source="daten.xml" XPath="/Root/Categorys/Category/Items/Item" />
</Grid.DataContext>

XPath="/Root/Categorys/Category[Name='Hardware']/Items/Item"

<?xml version="1.0" ?>
<Root>
  <Categories>
    <Category Name='Hardware'>
      <Items>
        <Item>Std. PC</Item>
        <Item>Server</Item>
      </Items>
    </Category>
    <Category Name='Software'>
      <Items>
        <Item>Winword</Item>
        <Item>Excel</Item>
      </Items>
    </Category>
  </Categories>
</Root>

//Root/Categories/Category[@Name='Hardware']/Items/Item