You need to provide a little more information.
What Distribution are you using (Debian, Mandriva, Suse, or whatever)next
Version like Debian 3-something Suse 10, 10.1 or the like
Then when does this occur when you first log in after a reboot, or after logging out and then trying to re-log in
Sometimes even your processor or amountof RAM might give someone a clue
Also how are you trying to install (From sources, using an RPM file or using a package manager)
It may sound like abunch of bull, but there are many distributions and not all of them use the same procedure to install files.
post all the information you can about your system even if it doesn,t make sense to you, and it will make it easier to help you.
For two very long winded examples of how to ask a question in away that helps you get an answer more easily see this two links.(copy and paste to your location bar )