How can I share only one window with TeamViewer?

Use Presentation mode.

In Teamviewer 6 you can change from Remote Control to Presentation.
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Click Invite Participants and you will get a text window with some canned text you can send to the the person you want to have watch.

Hello,

Please join the presentation on http://www.teamviewer.com/download/version_6x/TeamViewerQJ.exe Use the following data to log in:
ID: 357 621 583
Password: 6288

Regards,

Once you start the presentation you will have a menu on the right hand side.
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Under Application Selection you can un-check everything you want to hide. The Local Monitor window shows you what your viewers are watching.