With space enough on my harddisk after reinstalling Windows 7 I could easily make a new partition and install Debian Squeeze 64 bit.
Get a netinstall image from http://www.debian.org/devel/debian-installer/ and after the first installation of the bare essentials I did a apt-get install xorg kdm kdebase .
( instead of using kdebase you could also use kde-plasma-desktop).
Gnome users : apt-get install xorg gdm gnome-core.
Add the following lines to /etc/apt/apt.conf :