<p>I have a template in word (.docx) format and want to replace some placeholders in this template with my own data, do you know where I can find the right classes for this project? It would be nice for me to know the namespace. Are there any newer classes for handling word documents in the docx format (xml) than the com classes? Thank you for all your answers, I hope someone can help me with my problem.</p>

<p>greets </p>

<p>Sebastian</p>

The new office formats (docx, xlsx, etc) are zip files that contain a collection of xml files.  With that in mind you have a couple of approaches.
You can use the Open XML SDK located at
http://www.microsoft.com/downloads/details.aspx?FamilyId=AD0B72FB-4A1D-4C52-BDB5-7DD7E816D046&displaylang=en
You can unzip the docx file, do a search and replace for your tokens, and zip it back up.
There is a website at
openxmldeveloper.org
that is just for this kind of thing.  Also, bear in mind that they are already shipping a beta version 2 of the SDK.