Spybot-S&D will uninstall from the Windows Add/Remove Software control panel without problems.
If you want to completely get rid of Spybot-S&D and the Add/Remove does not help, you can delete the installation folder (usually C:\Program Files\Spybot - Search & Destroy\), then download
this very small fix and execute it (important! - this will remove all remains from the registry).
If you just want to upgrade to a newer version, please follow the same instructions like above and then install the new version.
After following these instructions please restart your system so that the changes can take place.