Two possibilities that I can think of: Either you have more than one copy of Inkscape on your computer, or Inkscape is not installed in the right location. So, first check to see if you might have one version of inkscape– for example, one where you unzipped the download, or on your desktop. If so, remove it.
Then, note that on Windows, Inkscape needs to be installed in the [Program Files directory]Inkscape directory before the installer will work correctly. If your copy of Inkscape is not there, move it there, and then run the installer again. You may also want to upgrade to the latest Inkscape (0.48.2) before installing the extensions.