Debian 6.0, like the one on the attached screenshot, then read on as this is very easy to fix. The problem is caused by an improper entry in /etc/fstab created by Debian installer: if you've installed Debian from a USB disk, then a "cdrom" entry pointing to that drive could have been created in fstab, e.g.
/dev/sdb1 /media/cdrom0 udf,iso9660 user,noauto 0 0
Just remove such entry and you're done! Note: I experienced this issue when installing Debian 6.0 iso. Chances are this issue was corrected by Debian 6.0.1a (or later) iso, but I haven't verified this.