I have seen this before when portions of the install are manually deleted (ie if you delete the National Instruments Folder in Program Files). If you have the original media that you used for installation, you can do a repair install or run the installer from the command line with the \reinstall flag which causes it to overwrite already installed files. At that point the software should correctly show up in Add/Remove Programs and you should be able to remove it from the computer.
The other way is to manually delete the files from Program Files and use a registry cleaner to delete the registry keys, but I don't recommend that.
Product Support Engineer | LabVIEW R&D | National Instruments | Certified LabVIEW Architect