If you install debian etch by default it will install gnome desktop.If you want to install KDE desktop environment you need to follow the following procedure
Install the KDE login manager and core KDE Components
#apt-get install kdm kde-core
at the time of installation it will promt you for which login manager you want to use as default one kdm or gdm select one of your choice and press enter this will complete the installation.
Now you need to logout from the existing desktop session to login KDE session.
Once you logout you should see similar to the following screen to login here you need to click on session
once it opens the session menu you need to select KDE and click on change session
Now you need to enter username and password to login and click on ok you should see the following screen here you need to select you want make this is your default session or only for this session
Now you should see the following screen to start configuring your KDE desktop first screen you need to select your country and language and click on next
Next one is system behaviour and click on next
Select Eyecandy-o-meter and click on next
Select your KDE desktop theme and click on next
The following screen shows you finish the setup by clicking on finish
KDE 3.5 is loading
Once it loaded fully you should see similar to the following screen as your KDE Desktop