Since i install those things from installation CD, I am thinking there are packages missing.
Anyone can give a help?
If your installation CD was good (with correct md5sum hash) there is less change that packages are missing. Have you check the CD integrity already?
Another possibility is to reconfigure your xorg.conf file. A driver or display reso might be in issue. Try using 'vesa' driver first and find out if the login window improves. '--# Xorg -configure'.
I am not a squeeze user. My experience is mainly based on slackware, BSD and other linux distros.
Hope it helps.