• Programs that are not uninstalled completely from your computer can leave files, folders, processes and registry entries behind on your computer. In many cases, the partial uninstallation does not cause any problems with the rest of the computer, but sometimes conflicts and issues arise from the files left behind.
  
  

Registry Entries

• When a program is installed on your computer, the installation package writes entries to the registry so that the program can function correctly. The registry entries generally do not interfere with operating system functionality. If the registry fills up with entries from programs that are no longer installed, corrupted or malfunctioning registry entries can cause problems.
  
  

Files and Folders

• A software package that does not have a properly functioning uninstaller can leave the program folders or files behind. If you run into many programs that have this issue, you could be dedicating hard drive space to programs that you don't even use anymore.
  
  

Installation

• One of the most significant problems with an incomplete uninstall is that if you try to install the program again in the future, the partial uninstall can interfere. The installer might not function correctly or it may think that you already have the program installed. The left over registry entries may also cause direct interference with the installer. This problem is not limited to the exact same program either, as updated versions or even other software made by the same company could encounter errors and problems.