Start the install process by double clicking Copasi-*-WIN32.msi or Copasi-*-WIN32.exe. If you do not have administrative rights an the installation to the default directory fails you must choose a directory where you have write permission.
CopasiUI is under the Start menu, CopasiSE may be started through the command line.
Please note, you will have to replace the * above with the build number of your package.