License problem after installing AutoCad 2011

I installed AutoCad 2011 with a single server license. After the installation I find it difficult to open AutoCad.
The following error message pops out.
Attached is a screenshot of the error message ("A Valid License could not be obtained by the license manager" error [-5.412.0]) so that you can get a much clearer view of the problem.
Please, you help is much valued.

Error:
A Valid License could not be obtained by the license manager.
If you are still unable to access a license, contact your system administrator.
Error [-5.412.0]
