* By default, GNS installs in the directory C:\Program Files(x86)\GNS3 directory. GNS3 installs Dynamips below that in C:\Program Files(x86)\GNS3\Dynamips. However, GNS3 is configured by default to look for Dynamips under C:\Program Files\GNS3\Dynamips. You will need to update the path to the correct location (Under Edit, Preferences, Dynamips).
* Check that you have the correct file system permissions to the above directories.