libdvdcsswhich is a simple library designed for accessing DVDs like a block device without having to bother about the decryption.
The instructions here are mainly from EeeUser Wiki: Watch DVD. I just add some screenshots for Linux novice.
1) Download the Debian package libdvdcss2_1.2.9-1_i386.deb (right click on this link and "Save Link as").
2) After your download is completed, open "File Manager", locate the file you have downloaded, right click on it, and select "Install DEB file".
3) Enter your password when prompted, then click [OK].
4) Wait until the installation is completed.
5) Open [Media Player] from the [Play] tab > Select [Options] > [Preferences]
6) Select "xv" as output video driver, then click on [OK].
Now your EeePC is ready to watch most DVDs on the default media player (smplayer).