This usually indicates that the .dll in your installation has been corrupted. I would suggest 2 possible fixes;
1. If you know of someone that has a good installation of the same level (I.e. version and service pack) simply copy and paste the .dll from theirs into yours (be sure to remove the old version of the .dll)
2. If you don't have access to another installation try either repairing or re-installing your SolidWorks installation.