The Guide shows you the Steps for Switching from KDE4 to Lxde Desktop on Debian Linux Wheezy.
To Login into the Lxde Desktop we will Install and Make Use of the slim Display Manager.
Finally, the post’s Content is Expressly Reduced to Give Focus to the Essentials Instructions and Commands.

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Installing the Lxde Desktop.
su
If Got “User is Not in Sudoers file” then see: How to Enable sudo
apt-get install aptitude tasksel
tasksel install lxde-desktop --new-install
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Installing and Switching to Slim Display Manager.
aptitude install slim
Choose the slim Display Manager.
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Disabling kdm Display Manager.
aptitude install rcconf
rcconf
Disable the kdm Display Manager.
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Setup Slim for Running a Desktop on Debian:
Debian Slim Setup How-to