There is a problem on showing purper desktop when using some ATI graphic cards. Here is one of the solutions I can find to solve this.

In the terminal, type the below command line by line to get the the open source ATI driver you needed for the problem.

sudo apt-get install build-essential autoconf automake libtool pkg-config git-core

sudo apt-get install libdrm-dev x11proto-gl-dev mesa-common-dev xutils-dev x11proto-xf86dri-dev x11proto-fonts-dev x11proto-randr-dev x11proto-video-dev x11proto-xext-dev x11proto-xinerama-dev x11proto-render-dev xserver-xorg-dev

git clone git://anongit.freedesktop.org/xorg/driver/xf86-video-ati

cd xf86-video-ati

sudo ./autogen.sh –prefix=/usr –enable-dri

sudo make

sudo make install