apt-get -s install -t unstable kscreensaver kdelibs4 libqt3c102-mt


And there will be a whole bunch more packages you will have to add the end of the line as it gives you the same warning about them. Apt is trying to upgrade your install to the latest KDE in unstable because that is the only version of the screensaver available you are best doing this from a console login from the KDE greeter screen it is not a good idea to upgrade KDE while it is running. Once you have the simulation of the install working without warning about the packages remove the -s in the line to actually install. To restart KDE from the console and get a new login screen use /etc/init.d/kdm restart.