Problem: ArcView (Single Use) Register Now button greyed out
After installing ArcGIS ArcView (Single Use), the Register Now button is greyed out in the Desktop Administrator under Register ArcView and Extensions.
Solution or Workaround
Ensure Internet Explorer 4.0 or higher is installed. Select About Internet Explorer from under the Help menu. If the version is not 4.0 or higher, download and install the current version from Microsoft.
Run RegisterFixed.exe found in C:\Program Files\ArcGIS\bin to start the registration process. Double-click on the file from within Windows Explorer.